This code can't actually be enabled for building and is incomplete, but it's
here because we know we want to support this at some point and there's not much
reason to have it in a separate branch if a simple #ifdef can disable it.
It was never fully implemented and will be replaced by OpenSubdiv. Only linear
subdivision remains now. Also includes some refactoring in the split/dice code,
adding a SubdParams struct to pass around parameters more easily.