LibOverride: Handle dependencies in both directions in partial override cases.

When creating etc. a liboverride based on a partial hierarchy
pre-selection (e.g: override hierarchy on the rig object of a
character), now all linked data also using that rig (e.g. all meshes
deformed by that armature) will also automatically be overridden.

This si achieved by following dependencies in the reversed order (from
used IDs to using IDs) when we find one tagged for override.

source/blender/blenkernel/intern/lib_override.cc

@@ -687,6 +687,51 @@ static void lib_override_group_tag_data_clear(LibOverrideGroupTagData *data)
   memset(data, 0, sizeof(*data));
 }
 
+static void lib_override_hierarchy_dependencies_recursive_tag_from(LibOverrideGroupTagData *data)
+{
+  Main *bmain = data->bmain;
+  ID *id = data->id_root;
+  const bool is_override = data->is_override;
+
+  if ((*(uint *)&id->tag & data->tag) == 0) {
+    /* This ID is not tagged, no reason to proceed further to its parents. */
+    return;
+  }
+
+  MainIDRelationsEntry *entry = static_cast<MainIDRelationsEntry *>(
+      BLI_ghash_lookup(bmain->relations->relations_from_pointers, id));
+  BLI_assert(entry != nullptr);
+
+  if (entry->tags & MAINIDRELATIONS_ENTRY_TAGS_PROCESSED_FROM) {
+    /* This ID has already been processed. */
+    return;
+  }
+  /* This way we won't process again that ID, should we encounter it again through another
+   * relationship hierarchy. */
+  entry->tags |= MAINIDRELATIONS_ENTRY_TAGS_PROCESSED_FROM;
+
+  for (MainIDRelationsEntryItem *from_id_entry = entry->from_ids; from_id_entry != nullptr;
+       from_id_entry = from_id_entry->next) {
+    if ((from_id_entry->usage_flag & IDWALK_CB_OVERRIDE_LIBRARY_NOT_OVERRIDABLE) != 0) {
+      /* Never consider non-overridable relationships ('from', 'parents', 'owner' etc. pointers)
+       * as actual dependencies. */
+      continue;
+    }
+    /* We only consider IDs from the same library. */
+    ID *from_id = from_id_entry->id_pointer.from;
+    if (from_id == nullptr || from_id->lib != id->lib ||
+        (is_override && !ID_IS_OVERRIDE_LIBRARY(from_id))) {
+      /* IDs from different libraries, or non-override IDs in case we are processing overrides,
+       * are both barriers of dependency. */
+      continue;
+    }
+    from_id->tag |= data->tag;
+    LibOverrideGroupTagData sub_data = *data;
+    sub_data.id_root = from_id;
+    lib_override_hierarchy_dependencies_recursive_tag_from(&sub_data);
+  }
+}
+
 /* Tag all IDs in dependency relationships within an override hierarchy/group.
  *
  * Requires existing `Main.relations`.
@@ -703,13 +748,13 @@ static bool lib_override_hierarchy_dependencies_recursive_tag(LibOverrideGroupTa
       BLI_ghash_lookup(bmain->relations->relations_from_pointers, id));
   BLI_assert(entry != nullptr);
 
-  if (entry->tags & MAINIDRELATIONS_ENTRY_TAGS_PROCESSED) {
+  if (entry->tags & MAINIDRELATIONS_ENTRY_TAGS_PROCESSED_TO) {
     /* This ID has already been processed. */
     return (*(uint *)&id->tag & data->tag) != 0;
   }
   /* This way we won't process again that ID, should we encounter it again through another
    * relationship hierarchy. */
-  entry->tags |= MAINIDRELATIONS_ENTRY_TAGS_PROCESSED;
+  entry->tags |= MAINIDRELATIONS_ENTRY_TAGS_PROCESSED_TO;
 
   for (MainIDRelationsEntryItem *to_id_entry = entry->to_ids; to_id_entry != nullptr;
        to_id_entry = to_id_entry->next) {
@@ -733,6 +778,15 @@ static bool lib_override_hierarchy_dependencies_recursive_tag(LibOverrideGroupTa
     }
   }
 
+  /* If the current ID is/has been tagged for override above, then check its reversed dependencies
+   * (i.e. IDs that depend on the current one).
+   *
+   * This will cover e.g. the case where user override an armature, and would expect the mesh
+   * object deformed by that armature to also be overridden. */
+  if ((*(uint *)&id->tag & data->tag) != 0) {
+    lib_override_hierarchy_dependencies_recursive_tag_from(data);
+  }
+
   return (*(uint *)&id->tag & data->tag) != 0;
 }