Alembic Procedural: precompute vertex normals

This precomputes vertex normals in the procedural and caches them in case none
are found in the archive. This only applies to polygon meshes, as subdivision
meshes are yet to be subdivided, so it is useless to do this computation.

The goal here is to speed up data updates between frames, as computing normals
shows up in profiles even for large objects. This saves around 16% of update time
for a production file.

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