Add simple support for rendering TileMap ImageLayers

Tiled allows for statically positioned ImageLayers that are literally just a layer with a single image representing its entirety.  This ensures they are loaded and can be rendered.

It's nothing really special, but the support is nice if you're someone like me who designs their maps largely using GIMP/Photoshop at sizes less than 4096x4096 and uses Tiled for managing multiple layers and collision.