Handle polygons with self-overlapping edges

[mourner]

Self-intersections are handled, but self-overlaps may cause trouble. Not fully clear where it fails (no test case yet), but Martinez responded:

Some time ago I were asked to extend the algorithm to cope with polygon with overlapping edges. When I devised the algorithm I didn't think these polygons were so common. I tried to find a solution keeping the algorithm simplicity, but it didn't find it. Anyway, I will try these days again. If I make some progress I will write to you.

Maybe we'll have some luck too. Or at least a practical workaround.

[mourner]

What makes this tricky is that I haven't been able to find a reproducible test with self-overlapping edges case that actually fails. Perhaps it fails in later stages of the algorithm (contour reconstruction), I'm not sure.