Isometric rendering more or less complete for single chunks

Now just have to enable it for region and world rendering.
Isometric region rendering works

Also:
- Added some optimizations to combine_alpha()
- Cleaned up #includes and added some guards against double inclusion
- Fixed a careless memory leak in isochunk.c
- Fixed a colour in the CSV that didn't show up on the map until
isometric rendering!
Merge and refactor isometric functions into main region/chunk files
Refactor world.c in preparation for isometric rendering
World isometric rendering works!