Introduce QgsTiledMeshIndex

This is an equivalent of the point cloud index class, with some
notable differences:

- The class is designed to be thread safe. There's a shallow copy
QgsTiledMeshIndex class, which contains an implicitly shared
QgsAbstractTiledMeshIndex object. Tiled mesh data providers will
be accompanied by a concrete class of QgsAbstractTiledMeshIndex.
- The QgsTiledMeshIndex class takes care of thread safety by
protecting the underlying QgsAbstractTiledMeshIndex via a read/write
lock
- Callers can request tiled mesh nodes from the index by calling
QgsTiledMeshIndex::getNodes along with a QgsTiledMeshRequest object.
This will return the tree structure of nodes matching the request.
Currently only geometric error based filtering is supported
by QgsTiledMeshRequest, but bounding box based filtering will
also be introduced.
- Clients can request tile content by calling
  QgsTiledMeshIndex::retrieveContent. This will either return
a cached version of the content (if available), or retrieve and cache
the content for future retrieval. Currently this uses a custom caching
mechanism, but this will be replaced in future with
QgsTileDownloadManager. (Currently QgsTileDownloadManager lacks support
for blocking gets which is required by the index to avoid use
of local event loops)

The api is implemented by the cesium tiles data provider, which
builds the index dynamically as required from the underlying
cesium tiles tileset.json definitions. (The node index is NOT built
upfront in order to minimize memory requirements for large
tilesets). Currently the cesium index implementation only handles
single-file tilesets, with support for sub tilesets to be introduced
later.

python/core/auto_generated/tiledmesh/qgstiledmeshdataprovider.sip.in

@@ -79,6 +79,17 @@ This corresponds to the root node bounding volume.
    Coordinates in the returned volume are in the :py:func:`~QgsTiledMeshDataProvider.meshCrs` reference system, not the :py:func:`QgsDataProvider.crs()` system.
 %End
 
+    virtual QgsTiledMeshIndex index() const = 0;
+%Docstring
+Returns the provider's tile index.
+
+This is a shallow copy, implicitly shared container for an underlying :py:class:`QgsAbstractTiledMeshIndex`
+implementation.
+
+The index is thread safe and can be used safely across multiple threads or transferred between
+threads.
+%End
+
 };
 
 /************************************************************************

python/core/auto_generated/tiledmesh/qgstiledmeshindex.sip.in

@@ -0,0 +1,79 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/tiledmesh/qgstiledmeshindex.h                               *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+
+
+
+
+class QgsTiledMeshIndex
+{
+%Docstring(signature="appended")
+An index for tiled mesh data providers.
+
+This is a shallow copy, implicitly shared container for an underlying :py:class:`QgsAbstractTiledMeshIndex`
+implementation.
+
+The class is thread safe and can be used safely across multiple threads or transferred between
+threads.
+
+.. versionadded:: 3.34
+%End
+
+%TypeHeaderCode
+#include "qgstiledmeshindex.h"
+%End
+  public:
+
+
+    ~QgsTiledMeshIndex();
+
+    QgsTiledMeshIndex( const QgsTiledMeshIndex &other );
+%Docstring
+Copy constructor
+%End
+
+    bool isValid() const;
+%Docstring
+Returns ``True`` if the index is valid.
+%End
+
+    QgsTiledMeshNode rootNode() const;
+%Docstring
+Returns the root node for the index.
+%End
+
+    QgsTiledMeshNode *getNodes( const QgsTiledMeshRequest &request ) /Factory/;
+%Docstring
+Returns a node tree containing all nodes matching the given ``request``.
+
+Caller takes ownership of the returned node.
+
+May return ``None`` if no data satisfies the request.
+%End
+
+    QByteArray retrieveContent( const QString &uri, QgsFeedback *feedback = 0 );
+%Docstring
+Retrieves index content for the specified ``uri``.
+
+The content is cached within the index, so multiple calls are potentially cost-free.
+
+The optional ``feedback`` argument can be used the cancel the request early.
+%End
+
+};
+
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/tiledmesh/qgstiledmeshindex.h                               *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

python/core/auto_generated/tiledmesh/qgstiledmeshrequest.sip.in

@@ -0,0 +1,77 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/tiledmesh/qgstiledmeshrequest.h                             *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+
+
+class QgsTiledMeshRequest
+{
+%Docstring(signature="appended")
+
+Tiled mesh data request.
+
+.. versionadded:: 3.34
+%End
+
+%TypeHeaderCode
+#include "qgstiledmeshrequest.h"
+%End
+  public:
+
+    QgsTiledMeshRequest();
+
+    double requiredGeometricError() const;
+%Docstring
+Returns the required geometric error treshold for the returned nodes, in
+mesh CRS units.
+
+If the error is 0 then no geometric error filtering will be applied.
+
+.. seealso:: :py:func:`setRequiredGeometricError`
+%End
+
+    void setRequiredGeometricError( double error );
+%Docstring
+Sets the required geometric ``error`` treshold for the returned nodes, in
+mesh CRS units.
+
+If the ``error`` is 0 then no geometric error filtering will be applied.
+
+.. seealso:: :py:func:`requiredGeometricError`
+%End
+
+    void setFeedback( QgsFeedback *feedback );
+%Docstring
+Attach a ``feedback`` object that can be queried regularly by the request to check
+if it should be canceled.
+
+Ownership of ``feedback`` is NOT transferred, and the caller must take care that it exists
+for the lifetime of the request.
+
+.. seealso:: :py:func:`feedback`
+%End
+
+    QgsFeedback *feedback() const;
+%Docstring
+Returns the feedback object that can be queried regularly by the request to check
+if it should be canceled, if set.
+
+.. seealso:: :py:func:`setFeedback`
+%End
+
+};
+
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/tiledmesh/qgstiledmeshrequest.h                             *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

python/core/core_auto.sip

@@ -714,8 +714,10 @@
 %Include auto_generated/tiledmesh/qgscesiumutils.sip
 %Include auto_generated/tiledmesh/qgstiledmeshboundingvolume.sip
 %Include auto_generated/tiledmesh/qgstiledmeshdataprovider.sip
+%Include auto_generated/tiledmesh/qgstiledmeshindex.sip
 %Include auto_generated/tiledmesh/qgstiledmeshlayer.sip
 %Include auto_generated/tiledmesh/qgstiledmeshnode.sip
+%Include auto_generated/tiledmesh/qgstiledmeshrequest.sip
 %Include auto_generated/sensor/qgssensormodel.sip
 %Include auto_generated/sensor/qgssensormanager.sip
 %Include auto_generated/sensor/qgssensorregistry.sip

src/core/CMakeLists.txt

@@ -342,10 +342,12 @@ set(QGIS_CORE_SRCS
   tiledmesh/qgstiledmeshconnection.cpp
   tiledmesh/qgstiledmeshdataitems.cpp
   tiledmesh/qgstiledmeshdataprovider.cpp
+  tiledmesh/qgstiledmeshindex.cpp
   tiledmesh/qgstiledmeshlayer.cpp
   tiledmesh/qgstiledmeshlayerrenderer.cpp
   tiledmesh/qgstiledmeshnode.cpp
   tiledmesh/qgstiledmeshprovidermetadata.cpp
+  tiledmesh/qgstiledmeshrequest.cpp
 
   sensor/qgssensormodel.cpp
   sensor/qgssensormanager.cpp
@@ -1910,10 +1912,12 @@ set(QGIS_CORE_HDRS
   tiledmesh/qgstiledmeshconnection.h
   tiledmesh/qgstiledmeshdataprovider.h
   tiledmesh/qgstiledmeshdataitems.h
+  tiledmesh/qgstiledmeshindex.h
   tiledmesh/qgstiledmeshlayer.h
   tiledmesh/qgstiledmeshlayerrenderer.h
   tiledmesh/qgstiledmeshnode.h
   tiledmesh/qgstiledmeshprovidermetadata.h
+  tiledmesh/qgstiledmeshrequest.h
 
   sensor/qgssensormodel.h
   sensor/qgssensormanager.h