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Alembic: support reading per-vertex UV sets

This adds support for importing UV sets which are defined per vertex,
instead of per face corners. Such UV sets can be generated when the
mesh is split according to UV islands, or when there is only one UV
island, in which cases only a single UV value can be stored per
vertex since vertices will never be on a seam.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D11584