Investigate / add support for OpenSubdiv

[snogglethorpe]

https://github.com/PixarAnimationStudios/OpenSubdiv

Last time I looked, this had some technical issues on Debian (some of which are apparently resolved):

• Add Debian library paths to cmake searches PixarAnimationStudios/OpenSubdiv#128
• Fails to find GLEW on Debian (althoug it's installed) PixarAnimationStudios/OpenSubdiv#127

It also has licensing issues:

PixarAnimationStudios/OpenSubdiv#126

Basically, it's licensed under the "MSPL", which is a gratuitously-GPL-incompatible license. It appears this was done inadvertently, not maliciously (but changing the license on an existing project is typically pretty difficult). The effects are unclear; some research shows that some people think the MSPL is GPLv2 incompatible, but GPLv3 compatible; the Blender people apparently consider OpenSubdiv unusable because of this.

• Check out the OpenSubdiv Google group about the licensing issue: https://groups.google.com/forum/#!forum/opensubdiv

[snogglethorpe]

OpenSubdiv seems to have been relicensed under the Apache 2.0 license, which is clearly GPL3 compatible. Nice. :]