Blender Git Loki

Subdiv: Initial implementation of topology cache

This commit makes it so OpenSubdiv's topology refiner is kept
in memory and reused for until topology changes. There are the
following modifications which causes topology refiner to become
invalid:

- Change in a mesh topology (for example, vertices, edges, and
faces connectivity).
- Change in UV islands (adding new islands, merging them and
so on),
- Change in UV smoothing options.
- Change in creases.
- Change in Catmull-Clark / Simple subdivisions.

The following limitations are known:

- CPU evaluator is not yet cached.
- UV islands topology is not checked.

The UV limitation is currently a stopper for making this cache
enabled by default.