Map 3d conversion WIP

What seems to work:
- Loading all map layers at once on game start
- Shifting all map layers at once when moving around

What doesn't work:
- There seems to be a strange offset of things on the lower z-levels. It's actually pretty hilarious, you can only drop down into the basement, and have to move back to the stairs which will spawn you into the wall of the evac shelter.
- Saving doesn't work, still need to track this bugger down. =(
I think the comment is wrong, or at least misleading, NUMBER_Z_LAYERS is the total number of z-levels, it does not have to be symmetric, OVERMAP_DEPTH could 20 and OVERMAP_HEIGHT could be 4.