Draft: Some FloorZ/CeilingZ function refactoring, and P_MobjFloorZ/P_MobjCeilingZ testing for deathpits

Hopefully resolves the concerns in the discussion of Issue #1150 (closed) .

Also, this removes P_CameraFloorZ/CeilingZ functions, because the only thing utilized from the mobj pointer was the ->radius, which can just be refactored into a separate function parameter. Now we can call FloorZ/CeilingZ functions without needing a mobj!