Blender Git Loki

LibOverride: refactor of relationships handling in library overrides.

First step towards a better handling of relationships between IDs in
override context, especially when a resync is needed.

First, introduce a new flag to override operations,
`IDOVERRIDE_LIBRARY_FLAG_IDPOINTER_MATCH_REFERENCE`, for ID pointers.
It keeps track of whether an RNA ID pointer has been kept to its
'natural overriden ID' (in override hierarchy context), or has actually
been re-assigned to some other data-block.

Second, refactor how we deal with relationships between IDs in override
hierarchy code, especially in resync case. This will fixe several cases
listed in T83811, especially the case where an ID pointer to an existing
override needs to be updated to a new one due to a matching change in
linked data.