Fix compositor backdrop gizmo refresh problem when toggling sidebar

The backdrop image gizmo was not following the backdrop image, it
needs to be refreshed whenever the view changes. The region init
callback is executed whenever the region size changes, so that should be
a reliable place to do that.

Reported as part of T87591.

source/blender/editors/space_node/space_node.c

@@ -651,6 +651,10 @@ static void node_main_region_init(wmWindowManager *wm, ARegion *region)
   lb = WM_dropboxmap_find("Node Editor", SPACE_NODE, RGN_TYPE_WINDOW);
 
   WM_event_add_dropbox_handler(&region->handlers, lb);
+
+  /* The backdrop image gizmo needs to change together with the view. So always refresh gizmos on
+   * region size changes. */
+  WM_gizmomap_tag_refresh(region->gizmo_map);
 }
 
 static void node_main_region_draw(const bContext *C, ARegion *region)