Implemented category fallback for tile rendering [depends SDL2]

For instance, "unknown_monster" will serve as fallback for monsters without sprites, and "unknown_monster_MAMMAL" will serve as fallback for species MAMMAL without sprite. Similarly. "unknown_item" will serve as fallback for items without sprite, and "item_unknown_CONTAINER" will serve as fallback for containers with no sprite.

This PR depends on the SDL2 PR to land first.