gdk: Make gdk_pixbuf_get_from_*() bindable
The ownership of the return value for gdk_pixbuf_get_from_window() and gdk_pixbuf_get_from_surface() was determined by the first argument. Because that is an ugly design and the functions are new to GTK3, we decided to adapt them. And that adaptation was quite easy since almost no one passses anything but NULL as the first argument.
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