Face-varying primvars are not interpolated correctly in the render delegate when the mesh is subdivided

[sirpalee]

Describe the bug
When using face-varying primvars with subdivision the primvars are not interpolated correctly, and none of the subdiv parameters affect the behaviour of the primvar. This happens due to the render delegate getting flattened primvars from Hydra.

We need to investigate if newer USD versions have a workaround for this.

[sirpalee]

A new API was added to 21.05 that allows us to correctly sample face-varying primvars.

Added SampleIndexedPrimvar() and GetIndexedPrimvar() scene delegate API to allow users to access unflattened primvars and their indices.