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.
b0c6c65e7b