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 this (and the next line) should be done before calling shift_layer. Shifting triggers loading of new submaps which puts the vehicles of those new submaps into vehicle_list and updates the vehicle cache with the new vehicles.

Currently this does not do anything:
`reset_vehicle_cache` calls `clear_vehicle_cache()`, so the call here is not needed.
`reset_vehicle_cache` than iterates over `vehicle_list` which has just been cleared, so nothing happens here at all.

Also note the previous order of execution (before this commit):
```
clear_vehicle_cache();
vehicle_list.clear();
...
map::loadn
map::update_vehicle_list
...
reset_vehicle_cache
```
Now it's
```
map::loadn
map::update_vehicle_list
...
clear_vehicle_cache();
vehicle_list.clear();
reset_vehicle_cache
```