This PR fixes several issues with the versioning that replaces the old
auto smooth flag with a modifier.
One issue is that the flag wasn't cleared in the initial versioning
code. That means some objects have the replacement modifier but their
meshes still have the flag set. The fix for that is to make the
versioning idempotent by trying to find an existing node group before
adding a new one. The versioning is now re-run on all objects to clear
the flags. Flags on all meshes are cleared too, even unused meshes.
That could cause loss of the auto-smooth when the mesh is linked from
a different blend-file, but that situation should be very rare.
Another issue was that the versioning wasn't run when linking objects.
That was simple to solve by adding the versioning where the proxy
versioning already existed for that case.
Finally, arguably the largest issue was that the the newly added node
groups were always added as local data-blocks. When linking, having
library data-blocks point to local data-blocks not in that library is
quite bad and breaks assumptions around Blender. This is solved by
having an auto smooth node group per library.
Resolves#119516, #119455, #119447
Pull Request: https://projects.blender.org/blender/blender/pulls/119539
5bfe6ad8f8