Fof slope crash fix

This fixes yet another software renderer crash, this time relating to sloped FOFs: sometimes the renderer thinks parts of FOFs where the top and bottom heights are the same height are actually the bottom going through the top (in other words a negative height). This causes problems when drawing normal walls around such FOFs since the game obviously doesn't expect negative heights anywhere along the FOF - before slopes, the game could just flip around the top and bottom heights automatically with no problems. This branch also should fix crashes for all genuine cases of negative FOF heights when slopes are involved, I suppose.

Hopefully this stops FuriousFox's SUGOI map crashing for the time being, all seemed fine when I tested it out myself.

See merge request !96