Blender Git Loki

Fix T76514: Invalid geometry in Alembic crashes Blender

Even though {T76514} is caused by invalid geometry, and thus technically
constitutes a bug in the software that created the Alembic file, I would
like Blender not to crash on importing such a file.

The error in the Alembic file consists of invalid mesh loops, where
consecutive loops refer to the same vertex. The `BKE_mesh_validate()`
can actually correct these errors, so this commit focuses on two things:

- Letting Blender survive the situation until the mesh is loaded, and
- Detecting the error so that `BKE_mesh_validate()` can be called only
when necessary. This ensures there is only a minimal impact on
performance when loading actually valid data.

Differential Revision: https://developer.blender.org/D7703

Reviewed By: JacquesLucke