app: fix update of group layers when size changes only trigger an offset change

In gimp_group_layer_update_size(), when the new size happens to be the
same as the old size, we need to call gimp_pickable_flush() on the
group projection so it is ready to be used for rendering the image,
just like in gimp_group_layer_stack_update().

app/core/gimpgrouplayer.c

@@ -1208,6 +1208,9 @@ gimp_group_layer_update_size (GimpGroupLayer *group)
            */
           gimp_projectable_invalidate (GIMP_PROJECTABLE (group),
                                        x, y, width, height);
+
+          /*  see comment in gimp_group_layer_stack_update() below  */
+          gimp_pickable_flush (GIMP_PICKABLE (GET_PRIVATE (group)->projection));
         }
     }
 }