cleanup mall part 1

[curstwist]

Summary

SUMMARY: Infrastructure "Mall mapgen cleanup: merge roof maps, clean up terrains. Split up mapgen into several files."

Purpose of change

Getting ready to do more work on the mall, so I wanted to clean it some to improve readability and navigation.

Describe the solution

Merged the roof maps, used palettes and did some terrain fixes and improvements.
Move the maps into a dedicated folder and split them up by z level.

Testing

spawned in world to check mapgen.

Additional context

I'm probably adding a basement to the mall, so I wanted to split these up by z level for my sanity.
I still need to look at the ground and 2nd floor for merging map opportunities (like the parking lots).

Future plans include: adding the basement with subway access, updating the ground floor, adding an NPC faction that makes use of update_mapgen.

[ifreund]

Thanks!