More DefaultUiCamera fixes (#17120)

# Objective

Found more excessive `DefaultUiCamera` queries outside of extraction.
The default UI camera lookup only needs to be done once. Do it first,
not per node.

---------

Co-authored-by: MichiRecRoom <1008889+LikeLakers2@users.noreply.github.com>

crates/bevy_ui/src/focus.rs

@@ -213,6 +213,8 @@ pub fn ui_focus_system(
         })
         .collect();
 
+    let default_camera_entity = default_ui_camera.get();
+
     // prepare an iterator that contains all the nodes that have the cursor in their rect,
     // from the top node to the bottom one. this will also reset the interaction to `None`
     // for all nodes encountered that are no longer hovered.
@@ -239,7 +241,7 @@ pub fn ui_focus_system(
             let camera_entity = node
                 .target_camera
                 .map(TargetCamera::entity)
-                .or(default_ui_camera.get())?;
+                .or(default_camera_entity)?;
 
             let node_rect = Rect::from_center_size(
                 node.global_transform.translation().truncate(),

crates/bevy_ui/src/picking_backend.rs

@@ -72,6 +72,8 @@ pub fn ui_picking(
     // For each camera, the pointer and its position
     let mut pointer_pos_by_camera = HashMap::<Entity, HashMap<PointerId, Vec2>>::default();
 
+    let default_camera_entity = default_ui_camera.get();
+
     for (pointer_id, pointer_location) in
         pointers.iter().filter_map(|(pointer, pointer_location)| {
             Some(*pointer).zip(pointer_location.location().cloned())
@@ -133,7 +135,7 @@ pub fn ui_picking(
         let Some(camera_entity) = node
             .target_camera
             .map(TargetCamera::entity)
-            .or(default_ui_camera.get())
+            .or(default_camera_entity)
         else {
             continue;
         };
@@ -189,7 +191,7 @@ pub fn ui_picking(
             let Some(camera_entity) = node
                 .target_camera
                 .map(TargetCamera::entity)
-                .or(default_ui_camera.get())
+                .or(default_camera_entity)
             else {
                 continue;
             };

crates/bevy_ui/src/widget/text.rs

@@ -268,10 +268,12 @@ pub fn measure_text_system(
 ) {
     scale_factors_buffer.clear();
 
+    let default_camera_entity = default_ui_camera.get();
+
     for (entity, block, content_size, text_flags, computed, maybe_camera) in &mut text_query {
         let Some(camera_entity) = maybe_camera
             .map(TargetCamera::entity)
-            .or(default_ui_camera.get())
+            .or(default_camera_entity)
         else {
             continue;
         };