Merge branch 'angles' into 'next'

Slopes and stuff

Adds support for slopes, slopes on FOFs, slopes on translucent FOFs, slopes on FOFs with holes in the flat, slope physics, dynamic slopes, vertex slopes, dynamic vertex slopes, and a ham sandwich to the game. Only for software mode right now, though. (OGL still gets the physics and the sandwich.) Some things still need to be done, but for now this can be merged in to be finished later.

Please make sure nothing in the vanilla game breaks before giving the thumbs up for this merge.

Since this doesn't merge automatically, if the code review turns out positive and nobody else has done it, I'll handle the merging.

See merge request !22