Set the shape of the widget, or, rather, it's window, to that of
the supplied bitmap. Notice how the window behind the example
window can be seen because of the rather odd shape the example window has.
GTK.Window( GTK.WINDOW_TOPLEVEL )->add(GTK.Label("A rather Oddly shaped\n" "Window\n" "Indeed\n" "Or what do you\nthink?\n" "This text\n" "should\n" "be long enough"))->shape_combine_mask( GDK.Bitmap(Image.image(100,100,255,255,255)->rotate(10,0,0,0) ), 20,20)

NOTE: The widget must be realized before this function can be used