Args: GDK.Image and GDK.Bitmap, respectively.
You can skip the mask. The mask is used to mask the drawing of the image
on it's background. It will not make the container transparent, though.
Use shape_combine_mask on the container with the mask for that.
There is no need to call 'set' with the same GDK.Image again if
you have modified the image. You must, however, use ->queue_draw()
to redraw the image. Otherwise the old contens will be shown until
an refresh of the widget is done for any other reason.
GTK.Image()->get()