We need to leverage our per-patch ptex indexing scheme in the EvalLimit API.

- replace ptex indexing with the FarPtexCoord structure as a way to pass per-patch
  ptex data to the shaders.

  We are replacing a vector<int> arranged as :
  int[0] : ptex face index
  int[1] : (u,v) as 16 bits encoding the log2 coordinate of the top left corner

  Instead we are now using a struct arranged as :
  int[0] : ptex face index
  int[1] : is a bit-field containing u,v, rotation, depth and non-quad

  The u,v coordinates have been reduced to 10 bits instead of 16, which still
  gives us a lot of margin.

- Replace OsdVertexBufferDescriptor with something more adequate for general
  primvar representation (this name will probably eventually change...)

- Improve OsdPatchDescriptor
    - add a "loop" boolean (true if the patch is of loop type)
    - add a GetPatchSize() accessor

- OsdPatchArray :
    - remove some redundant elements (still more to do there)

- Fix all shader / examples / regressions & stuff to make this all work.

fixes #143