This uses the recently introduced evaluator's vertex
data to smoothly interpolate original coordinates instead
of using linear interpolation.
The orcos are interpolated at the same time as positions
and as such, the specific subdivision routine for the
orco extractor has been removed. The patch evaluation
shader uses a definition to enable code specific to
orco evaluation.
Since the orco layer may not have been requested on first
render, and since orco data is now stored in the OpenSubDiv
evaluator, the evaluator needs to be recreated if an
orco layer is suddenly available. For this, a callback
to check if the evaluator has the data was added. This is
added to the evaluator as the `Subdiv` cache stored in the
modifier is invalidated less often than the Mesh batch cache
and so leads to fewer evaluator recreations.
Differential Revision: https://developer.blender.org/D14999
9d9f2f1a03