Bug Description: In a GTK+ application with a menu bar, clicking the
menu item displays a dialog. However, when the user opens this dialog
and drags the parent window's menu bar with the cursor, the dialog
gets moved.
Issue: When _gtk_gesture_update_point calls the _update_widget_coordinates
function, the local variables x and y are not explicitly initialized, leading
to arbitrary values. For instance, in my case, x was 32767 and y was
-145750164 . These values are used in the subsequent call to
gtk_widget_translate_coordinates. In gtk_widget_translate_coordinates,
if ancestor is NULL, the function returns FALSE, and dest_x and dest_y
are not updated. The incorrect values of x and y cause data->widget_x
and data->widget_y to be incorrect, ultimately leading to abnormal
x and y values in the gtk_gesture_drag_update function.

To avoid this, we should set x and y to values clearly outside the widget.

Signed-off-by: Li Kai <likai@kylinos.cn>