Modified weighting of sharpness values for Chaikin crease method:
    - semi-sharp edges prevented from decreasing by averaging with inf-sharp
    - averaging now includes only semi-sharp edges around the vertex
    - see regression/shapes/catmark_chaikin2
Merge branch 'dev' of github.com:PixarAnimationStudios/OpenSubdiv into dev
Merge branch 'dev' of github.com:PixarAnimationStudios/OpenSubdiv into dev