texture_id should preallocate the id for textures that aren't loaded

[setzer22]

Currently it seems comfy returns a "missing" texture it whenever you call texture_id for a sprite that is currently being loaded. When you try caching these ids instead of invoking texture_id each frame, this gets inconvenient, because that'll cache the "missing" texture value and you'll keep drawing that each frame.

I think the whole thing would be more ergonomic if comfy instead preallocated texture ids when you start loading them, and then returns the right value immediately. Trying to draw a texture id that is not loaded yet, would be replaced by the "missing texture" transparently by the renderer.