Reuse Greiner–Hormann for 2D polygon clipping.

Greiner–Hormann is preferable to Sutherland-Hodgman, because it doesn't
add extraneous lines along the clip edge, and can even handle
self-intersecting polygons.  The line segments are generated via the
efficient Liang–Barsky line clipping algorithm, and Greiner–Hormann
rejoins them appropriately along the clip edge.

Still in progress: correct marking of whether an intersection is an
entry or exit point, and detection of points that lie exactly on the
clip edge, which should be jittered.
Add point-in-polygon test for 2D polygon clipping.
Fix interpolation.
Fix bug in Liang–Barsky clipping.