Image pooling for human unconscious/oxygenloss/fire&bruteloss screen overlays.
Tested and Fixed #826

Conflicts:
	code/game/objects/items/stacks/stack.dm
Refactor get_turf proc.
exploit fixes

Conflicts:
	code/game/machinery/Sleeper.dm
	code/game/machinery/cryo.dm
	code/game/mecha/mech_fabricator.dm