Face-varying primvars flattened during import

[sirpalee]

Current usdKatana flattens face-varying primvars during import, meaning there will be a unique value for each face-vertex.

This causes few issues:

• Potentially increased memory usage in the scenegraph. Face-varying representation can save memory when values are shared between vertices.
• Increased memory usage when rendering with KtoA. Arnold requires indices for face-varying values all the time, so KtoA needs to generate an index list for these values.
• When subdividing with Arnold, the subdivision code is not able detect shared UVs, so the subdiv_uv_smoothing value (that supports pinning uvs to vertices, edges etc.) is working incorrectly compared to Alembic.

Here is a screenshow showing the difference between a mesh imported from usd vs alembic, and when uv pinning is enabled for subdivision and rendered using KtoA. Left is usd, right is alembic.

Originally reported in: https://community.foundry.com/discuss/post/1195106/content/121275 .

[JamesPedFoundry]

This has been merged in as part of the changes in v19.11_fn4