Using a more specific type is more correct.

layer_water is the first layer that loads its sprites on its own.

This is much easier to work with when drawing many sprites with offsets.

I also moved LAYER_WORKERTASK to a very similar class. Some
deduplication will be needed.

Since many layers now load their sprites on their own, this lets us do
it for all layers at once.

Multiple layers need to load sprites to represent unit activities.
Reduce code duplication by loading them in an abstract base class.

So far only create the class. Tile loading and drawing will come next.

This is necessary to port the roads layer drawing to layer_roads.

This is a very deep refactor of the algorithms. The main change is the
use of std::bitset to replace bool[], which provides a few helpful
algorithms out of the box. Better code organization also helped reduce
its complexity significantly.

This function used to drive the drawing of mapview layers, but has now
been fully replaced by layer classes.

One variable is easier to work with than two.

Instead of trying to match something (with __ or _-_), skip the entry
for the alt graphic.

Roads rely on having terrain information available, but the server sends
terrains after rulesets. This moves extra initialization to the very
end, in handle_rulesets_ready(). All tileset loading code shoud probably
be done there.

This wraps load_sprite and specifies the tileset automatically, which is
a nice syntactic sugar in layer classes.

It's a low-level function whose usage should ideally be limited to
tilespec.cpp.

Fixes compilation with libc++.

I thought I could remove a warning about an optional sprite not being
found but it doesn't seem to be the case.

When loading many optional sprites, it's better to log a single line
summarizing everything than one line per sprite.

This will allow silencing them.

Also remove "warning" from the message itself.

Gate displaying these messages behind the --debug-tileset option. This
reduces the client verbosity.

Looks bad with amplio2.