A function of this type is responsible for freeing the pixel array of a pixbuf.
The gdk_pixbuf_new_from_data() function lets you pass in a pre-allocated
pixel array so that a pixbuf can be created from it; in this case you
will need to pass in a function of type
GdkPixbufDestroyNotify so that
the pixel data can be freed when the pixbuf is finalized.