Anim: add reverse pointers from bone to collection on overrides

When inserting a bone collection via a library override, make sure that
the reverse pointers (from the bones in the collection to the collection
itself) are also added. These are used for the "given the active bone,
which collections is it in?" kind of queries.

source/blender/animrig/intern/bone_collections.cc

@@ -73,6 +73,20 @@ void ANIM_bonecoll_free(BoneCollection *bcoll)
   MEM_delete(bcoll);
 }
 
+/**
+ * Construct the mapping from the bones to this collection.
+ *
+ * This assumes that the bones do not have such a pointer yet, i.e. calling this
+ * twice for the same bone collection will cause duplicate pointers. */
+static void add_reverse_pointers(BoneCollection *bcoll)
+{
+  LISTBASE_FOREACH (BoneCollectionMember *, member, &bcoll->bones) {
+    BoneCollectionReference *ref = MEM_cnew<BoneCollectionReference>(__func__);
+    ref->bcoll = bcoll;
+    BLI_addtail(&member->bone->runtime.collections, ref);
+  }
+}
+
 void ANIM_armature_runtime_refresh(bArmature *armature)
 {
   ANIM_armature_runtime_free(armature);
@@ -83,11 +97,7 @@ void ANIM_armature_runtime_refresh(bArmature *armature)
 
   /* Construct the bone-to-collections mapping. */
   LISTBASE_FOREACH (BoneCollection *, bcoll, &armature->collections) {
-    LISTBASE_FOREACH (BoneCollectionMember *, member, &bcoll->bones) {
-      BoneCollectionReference *ref = MEM_cnew<BoneCollectionReference>(__func__);
-      ref->bcoll = bcoll;
-      BLI_addtail(&member->bone->runtime.collections, ref);
-    }
+    add_reverse_pointers(bcoll);
   }
 }
 
@@ -143,6 +153,7 @@ BoneCollection *ANIM_armature_bonecoll_insert_copy_after(bArmature *armature,
 
   BLI_insertlinkafter(&armature->collections, anchor, bcoll);
   bonecoll_ensure_name_unique(armature, bcoll);
+  add_reverse_pointers(bcoll);
 
   return bcoll;
 }