Software crashes fix... fix

Fixes a typo introduced by merge request !75 (merged) that caused upper textures to set the wrong ceiling clipping value when not visible, allowing all sorts behind the walls to be visible. This is most noticable in GFZ2 when the inside of the tunnel section is visible

...probably a good idea to make sure this one doesn't introduce any MORE visual glitches by mistake (again, compare with 2.1.15 if possible)