The idea of this commit is to make it so we can enable OpenSubdiv by default
for the release builds but keep it limited to the viewport only for a specific
meshes. This is a temporary solution for until all the needed features are
supported on the OpenSubdiv side.
Flag itself is done as a dedicated field in modifier DNA so we can easily
remove it in the future without ending up with some temporary flag hanging
around forever.
7667ad2d4e