window: Cancel window grab ops on TOUCH_CANCEL events

Window dragging should be cancelled when the touch sequences we're using
are no longer available. Also listen to TOUCH_CANCEL events if the
window is grabbed and cancel the grab op when a TOUCH_CANCEL event
happens.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/427>

src/core/window.c

@@ -6565,6 +6565,10 @@ meta_window_handle_mouse_grab_op_event  (MetaWindow         *window,
         }
       return TRUE;
 
+    case CLUTTER_TOUCH_CANCEL:
+      end_grab_op (window, event);
+      return FALSE;
+
     default:
       return FALSE;
     }