Newer OpenSubdiv brings fixes and improvements for non-manifold meshes,
which fixes some crashes we've experienced in the recent past when using
Gregory patches.
Additionally, thing new version of OpenSubdiv brings sparse patches,
which allows to multi-thread topology refinement step.
Note these are intended for platform maintainers, we do not intend to
support users making their own builds with these. For that precompiled
libraries from lib/ should be used.
Implemented by Martijn Berger, Ray Molenkamp and Brecht Van Lommel.
Differential Revision: https://developer.blender.org/D2753