Merge branch 'opengl-slope-FOF-lighting-fix' into 'master'

OpenGL slope FOF lighting fix

This fixes some issues with sloped FOFs that affect lighting in OpenGL (as in, those that cast a shadow or have a colormap). Particularly, they can do strange things to any wall textures adjacent to them, as we've noticed ourselves in levels for 2.2. =P

See merge request !194