split less

[morganherlocker]

Right now we split tiles into 4 parts at each recursion. We really only need to split by the vertical and the horizontal, then the corner can be extrapolated. We also can use a modified Sutherland-Hodgman clip instead of a full Weiler-Atherton, since the tile is always concave.

[morganherlocker]

Irrelevant now that tile-scanline is in master