Blender Git Loki

Made evaluator persistent in memory

Now OpenSubdiv evaluator descriptor will only be re-created
if mesh topology changes.

Apparently don't see huge speed raise with CPU evaluation,
but this change lands a bit of a basis for further changes.

Requires changes made in OpenSubdiv's C-API which are currently
only there:

https://github.com/Nazg-Gul/OpenSubdiv/tree/dev