Modify Far remapping of singular vertices to point to their source vertex.

- Add a vector of index pairs to HbrMesh to track the index of a split
  vertex and its origin vertex
- Correct the Far remap tables in FarSubdivisionTablesFactory to point split
  vertices to their origin instead of themselves
- Fix regression/common/shape_utils.h to use the new HbrMesh::GetSplitVertices()
  method.
- Fix the osdPolySmooth example to use the new HbrMesh::GetSplitVertices()
  method.
- Add a paragraph to the documentation

fixes #241