Blender Git Loki

OpenSubdiv: Add explicit storage for mesh topology

The idea is to use this explicit storage for topology comparison rather
than using base level. While this will have memory overhead it allows
to simplify comparison of such things as:

- Vertex sharpness (where base level from topology refiner will have it
refined, meaning it will be different from what application requested
for non-manifold and corner vertices).

- It will allow to simplify face-vertices comparison, where currently
O(N^2) algorithm is used due to possible difference in face winding.

- It will also allow to avoid comparison-time allocation of edge map.

Currently no functional changes, just preparing for development which
will happen next.