Underground map attempt

A failed attempt at an underground map. Tried to use the existing region
system just adding a y value to render at.

Problems:
* Range is too far, feels cheaty even though you can't see through
walls.
* No good method to automatically switch between underground map and
surface map.
* Expensive to re-render visible regions when player y value changes.
* Trying to save the underground map as a single layer image does not
work very well. There are too many places that caverns overlap.

I'm going to remove most of this commit in the next commit and instead
just upgrade the old underground map :(

Maybe it will be useful as a branch point later on.