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
3e77f747c0