3D rotations are not commutative, so we must perform an inverse rotation
to move something from origin [0, 0] to [λ, φ]. Normally we are moving
origin [λ, φ] to [0, 0].
This fixes various problems that were occurring due to winding numbers
being tricky on a finite sphere.
This also includes some cleanup and fixes for degenerate points.
1. We no longer perform an equality check on GeoJSON polygon endpoints,
since we know that the first and last points should be equal.
2. However, there are special cases where a polygon just touches the
clip edge, generating two coincident intersection points. Here we avoid
generating a coincident point, and continue as normal.
3. Another special case is where we have coincident intersection points
e.g. due to a self-intersecting polygon. Here we continue calculating
the winding number as if this is a closed polygon (so we know whether to
insert a polygon around the whole clip edge).
Mike Bostock
Jason Davies