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Alembic export: export UV maps on every frame

Export UV maps to Alembic on every frame. This makes the export of UV
maps consistent with mesh normals. In the case of static UV maps it may
cause a slight slowdown (since they're exported on every frame now), but
due to the deduplication performed by the Alembic library, the resulting
files will be the same size anyway.

Thanks to @CodyWinch for providing the solution to the problem, and
writing the original patch D8397.

Differential Revision: https://developer.blender.org/D8397