Detect texture size from assets

[alice-i-cecile]

Setting the texture size manually (on a per-layer basis) is somewhat error-prone, and silently results in strange errors and aberrations if the user gets it wrong.

Ideally, this would be detected automatically based on the tile assets chosen (and the material handle passed in).

[alice-i-cecile]

Trying my hand at this, this is surprisingly involved / frustrating. The core issue is that we're working with Handle<ColorMaterial>> in most places, and the size is stored on the Texture currently.

This means that we're forced to use several layers of indirection, piping everything through the entire logic stack and further bloating things like MapQuery to store more Assets resources which we use to look up the information we need.

In the new renderer, this information will be stored on a new Image type: see bevyengine/bevy#2632.

My initial work can be found at https://github.com/alice-i-cecile/bevy_ecs_tilemap/tree/detect-size; anyone who's interested can pick this up / poke around and work off my branch freely, but I'm going to put this down for now and try to clean up the architecture a bit / wait for the rendering rework to drop.

[StarArawn]

This is now reality! 👍