Add functionality for detecting polygon-polygon intersections.

[rmrsk]

This functionality uses a BVH structure for detecting whether or not a set of input polygons intersect the polygons of another mesh. This is normally used for detecting self-intersections.

TODO:

• Add face-face intersection test
• Add BVH traversal functionality to mesh distance functions
• Add a known (bad) self-intersecting mesh (e.g., Klein bottle)