`OpenSubdiv_Buffer` is a wrapper that was introduced at the time
that Blender couldn't use CPP directly. It contains a pointer to
a VertBuf and callbacks to use GPU module on that buffer.
This PR replaces OpenSubdiv_Buffer with `blender::gpu::VertBuf` and
removes the wrapper.
NOTE: OpenSubdiv tests are added to blender_test executable to make the
library dependencies not to complicated.
Pull Request: https://projects.blender.org/blender/blender/pulls/135389
4a95c0405c