Replace the C-class pattern function pointers with actual class methods.
Other than the obvious benefit of not requiring the "this" pointer to be
explicitly passed into every function call, this will make it much simpler
to remove the entire C-API class and replace it with its "impl" next.
For that next step we need to expose code to the implementation
of the topology refiner, so instead of defining stubs locally in the
opensubdiv intern class, we spread some WITH_OPENSUBDIV checks
in the blenkernel. As far as I know this is the only way to remove the
intermediate C-API and call opensubdiv functions directly from there.
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