toPolygon2D() now use Clipper's cleaning algorithm

The old polygon simplification algorithm in toPolygon2D() was broken.
It could create self-intersecting paths which made CGAL barf and
resulted in non-manifold meshes. It has been replaced with the one
built into Clipper. I am not sure Clipper's is guaranteed to be
correct either, but it at least seems to have fewer fail cases.