* Fix: Blobs respect gravity if not otherwise prevented.
Tweak: Blobs do not directly interact with stairs
Tweak: Blobs, when expanding against a stair's middle structure, can grow up Z-levels if the turf allows.
Tweak: Blobs, if controlled by a player, may grow upward or downward, if the turfs allow.
Fix: Blob Overmind is no longer affected by gravity.
* Fringe case deleted blobs are null-loc'd for GC so they are not stuck in-map.
Railing checks properly function.
