Implement anchor slope rendering

Closes #5

This MR implements anchor slope rendering for thing types 777 and 778.

I think there's some cache things that could be ironed out, visual oddities that fix themselves after moving the anchors in Visual Mode, or having to re-enter Visual Mode twice, like with some midtextures that don't render for that first time. Also, with anchors directly on top of linedefs or vertices, they can behave very strangely.

I won't be fixing where the anchor things render in the visual world, since I think there's a bigger problem that needs to be dealt with first, but I'll accept any changes to allow the anchor things to render in more intelligent ways before a merge.

out