Merge branch 'opengl-planes' into 'master'

OpenGL planes fix

This branch removes a specific hack in the OpenGL code for detecting if a plane is for the floor or ceiling of a sector. This it turns out fixes ceiling slopes in Boinciel's SUGOI map going missing. Though, It doesn't fix that other glitch with one FOF (must be unrelated).

If you want to test these fixes out properly, make sure working in-level sectors, FOFs AND also polyobjects all still work fine, sloped or not.

See merge request !97