Well, I did not expect it to work. It seems that Gtk handles transparent drawing. So in fact, BGRABitmap could be modified so that it would do this when parameter is at False, and it should be modified so that with parameter True, it would fill the background as well.
In the meantime, it is surely faster with True for now. We need to modifiy both at the same time. In BGRABitmap I should do the change and in your library, you should put False as a parameter.