Reorder the order of faces reported in Frustum::ToPolyhedron() so that it matches the order of planes in Frustum::GetPlane. Fix Frustum->Polyhedron conversion when the Frustum is left-handed.
Clarify Frustum plane convention in documentation.
Add new functions:
  - PBVolume::ToPolyhedron generates a new convex Polyhedron that represents the same set of space as the plane-bounded volume does.
  - PBVolume::SetIntersection computes the set intersection between two PBVolumes.
  - Polyhedron::SetEquals tests if two Polyhedrons effectively represent the same volume of space.
  - Polyhedron::SwapVertices swaps two vertices around.
  - Polyhedron::CanonicalizeFaceArray eases comparing two Polyhedrons.
  - Polyhedron::ContainsFace tests if a particular face is specified in Polyhedron.
  - Polyhedron::FindClosestVertex searches for the closest vertex on a Polyhedron to the given target point.

Also add tests.