This switches evaluation of vertices which are on the boundaries
of PTex faces to a single threaded one. While this introduces
some slowdown it fixes ambiguity of PTex index used to evaluate
particular vertex.
Possible alternative solutions would be:
- Do some pre-calculation of index, then do evaluation in threads.
- Try using Gregory patches and see if that makes any affect.
Fix T60235: Flickering of object instances
8e35a968c5