Blender Git Loki

Store edge topology explicitly in Alembic files as a user property.

This is not a native part of the core Alembic mesh schema, but can be
defined simply as a custom property.

Two advantages:
* Loose edges without faces are stored in the Alembic files and restored
on loading
* Building edges on the fly after loading topology from the cache was
the biggest bottleneck for cache performance.