wm: fix multi-monitor configuration with different scale factors

Or at least try to do so...

#167

wm.js

@@ -445,8 +445,8 @@ var WindowManager = GObject.registerClass(
             );
 
             if (!prev_target_rect || !this.current_target_rect.equal(prev_target_rect)) {
-                this.notify('target-rect');
                 this._update_window_geometry();
+                this.notify('target-rect');
             }
         }