Revisit singular vertex fix for FarSubdivisionTables

The previous fix pointed far indexing tables to the origin vertex
of duped singular verts.

This fix goes one step further and actually shifts all vertex indexing
to start at the end of the coarse mesh vertices, using the space for
data that was previously occupied by duplicated singular verts.

The consequence is that client code no longer needs to duplicate vertex
data in vertex buffers (huzzah !).

- fix FarSubdivisionTablesFactory to shift factory vertex table offsets using Hbr's
  singular verts map
- fix schema table factories (Catmark, Loop...) to correctly use these offsets
- remove vertex data duplication code from osdPolySmooth example
- remove some (unrelated) cruft from glViewer example
- shape_utils unfortunately still needs to dubplicate the singular verts to
  allow the coarse edge drawing in our example viewers to work correctly
  (although it could be fixed to avoid data duplication too...)

fixes #241