USDZ Exporter: Some small fixes

[alexchuber]

• Retrieve Normal vertex buffer
• Retrieve correct UV set indices
• Add clearcoat support for PBR materials
• Allow export with an odd number of negative scales (aka flipped orientation). Not sure if it is the ideal fix, but my rationale is
  • This is what Three does, and seems to work OK.
  • This piece of documentation (last paragraph of https://openusd.org/release/api/usd_geom_page_front.html#UsdGeom_WindingOrder) seems to suggest that handling the orientation flip can be a run/load-time operation? Or, at least, I can't find anyplace where it says that this must be done at export-time. Someone please correct me if I'm wrong :)

[bjsplat]

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[bjsplat]

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[bjsplat]

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[bjsplat]

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