In previous code, the owner ID info would not be available when
processing an embedded ID in two cases, and was incorrectly set to the
processed (embedded) ID instead:
1. When directly calling `BKE_library_foreach_ID_link` on an embedded ID.
2. When using recursive processing (`IDWALK_RECURSE`).
This commit mostly fixes both cases, by using `BKE_id_owner_get` to find
the owner ID when it is unknown.
There are some caveats here though: in a few specific cases (mainly ID
copying, and depsgraph ID copying), `BKE_library_foreach_ID_link` can be
called on embedded IDs which owner ID is not yet valid. In such case, a
new flag can be used to keep using the previous behavior
(`IDWALK_IGNORE_MISSING_OWNER_ID`).
Fixing the issue with copy code being unaware of the owner ID when
copying an embedded one should also be fixed, but this will be addressed
separately.
Note that as 'side efect', this commit also fixes a matching issue in
the `lib_remap` code, where the `IDRemap.id-owner` pointer would also
wrongly be set to the remapped embedded ID instead of its actual owner.
This change is not expected to have any effect in current codebase.
36c3504994