Fix T91711: Blender 3.0 - The Rain demo scene breaks (Proxy to Override auto conversion).

Proxy conversion is a fairly particular case of liboverride creation, in
which remapping all local usages of linked data probably makes more
sense, rather than only doing so whitin the overridden 'group' of IDs.

source/blender/blenkernel/intern/lib_override.c

@@ -1000,6 +1000,18 @@ bool BKE_lib_override_library_proxy_convert(Main *bmain,
 
   DEG_id_tag_update(&ob_proxy->id, ID_RECALC_COPY_ON_WRITE);
 
+  /* In case of proxy conversion, remap all local ID usages to linked IDs to their newly created
+   * overrides.
+   * While this might not be 100% the desired behavior, it is likely to be the case most of the
+   * time. Ref: T91711. */
+  ID *id_iter;
+  FOREACH_MAIN_ID_BEGIN (bmain, id_iter) {
+    if (!ID_IS_LINKED(id_iter)) {
+      id_iter->tag |= LIB_TAG_DOIT;
+    }
+  }
+  FOREACH_MAIN_ID_END;
+
   return BKE_lib_override_library_create(bmain, scene, view_layer, id_root, id_reference, NULL);
 }