Support layered tile maps

[codypiersall]

MapStack supports layers of maps, which makes it possible to create some really cool looking maps. It would be nice if vismap natively supported layers of tiles.

Probably the layering would be done with pillow, so that the TileProvider could still just return a single rgb array... or maybe the returned rgb array should just gain another index. Not sure which is better at this point; if pillow doesn't do the layering, we'll have an issue where the Canvas._images map will need to change the keys or values to accomodate having several images associated with a single key.