Assorted Bug Fixes and Tweaks

Overmap Changes:
Updated overmap::open to use overmapbuffer
Updated overmap::good_river and overmap::good_road to not polish overmap edges, so they can properly connect
Updated overmap::draw_overmap to return tripoint, allowing teleporting across z-levels using debug menu
Fixed overmap special bugs:
"by_hiway" location specials no longer count bridges as roads
Large buildings rotate properly
Ponds no longer spawn in the middle of large buildings (in theory)
Roads should connect in the right direction
Specials should no longer be cut off by overmap edges

Overmap Buffer Change:
Updated overmapbuffer::get_existing to pull world filename without relying on the 0,0 overmap to exist

Mapgen Change:
Fixed bug in apartment tower rotation for the mod variant