Blender Git Loki

LibOverrides: First stage of detection of 'need resync'.

We can fairly easily detect some resync-needed cases when applying the
overrides operations on a Pointer RNA property.

This should cover all cases where an existing override's ID pointer is
changed in its linked data.

We still have to add code to detect when a not-yet-overridden linked ID
needs to become overridden (because its relations to other data-blocks
changed in a way that requires it).