Space transitions/Away missions loading fixes

Final pass at fixing Space Cube/space transitions/away missions loading :

* The Space Cube is now generated at world start, before away missions are loaded
This means away missions won't have space transitions

* The _setup_map_transition_ proc is protected from excessive tiles proceeding (the proc is set to the background if too much tiles are processed)

* Added a comment explaining the Space Cube, by Incoming

* Fixes #4903 : "mobs don't maintain momentum on z-level transition"

* Serendipitiously fixes #2614