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. =(
You're setting the absolute coordinates of the map in each call to this function, that means abs_sub ends up being completely wrong, ~~`set_abs_sub` should be called exactly once from `map::shift`~~. You're calling it from `map::shift`, it should not be called here.

Also note that `set_abs_sub` is called in `map::loadn` when gridx==0 and gridy==0 (the submap at `grid[0]`). Now it's called several times (with gridx==0 and gridy) for each z-level.