yay gravity fixes.
pixel fix for pathfinding thing
fix some broken walls in alien bases and abductors
fix sanity check
ammendments to facility destruct sequence
tidyin
fix typo

thanks to cfailde for pointing this out.
some explosion tweakage

don't start any explosions at a frame above 0, instead, range the starting
frame between 0 and -4 (meaning a 4 frame delay) and layer the explosions
accordingly. distribution should be more or less equal between these frame
delays.