Abyss slab

[rainlizard]

Here's the Abyss slab, requested by Logan and others, which War for the Overworld probably originated the idea for.

Creatures die when they fall in, they visually fade into the darkness and play their dropped scream sfx. You can slap them in, or anything that pushes a creature. You can possess and fall in, fly over it, see past it to aggro enemies, and build bridges (if enabled in terrain.cfg). Creatures should hopefully pathfind around it without ever accidentally falling in but this needs further testing. Boulders fall in too, although it seems they currently roll over one-slab gaps (changing boulder physics might affect other maps).

Alternative behaviour can added to rules.cfg if necessary.

Appearance could be improved, I'm relying on cube sides (which looks good half the time) and lighting falloff to obscure the deeper parts of the slab walls.