[ZScript] ChangeHeight() analog for LevelPostProcessor

[Josh771]

Sector.FloorPlane.ChangeHeight() is inaccessible since FloorPlane is read-only, and using the MoveFloor() and MoveCeiling() functions create undefined behavior from LevelPostProcessor.

Could we possibly get a simple function for moving a sector's floor or ceiling instantly -- and that being accessible from LevelPostProcessor? Can't really change sector heights at all, which is vital to a little map mutator I'm making right now.

[_mental_]

It's already in, just named differently.