Specifies the type of the function passed to
gdk_pixbuf_save_to_callback(). It is called once for each block of
bytes that is "written" by gdk_pixbuf_save_to_callback(). If
successful it should return %TRUE. If an error occurs it should set
@error and return %FALSE, in which case gdk_pixbuf_save_to_callback()
will fail with the same error.
Specifies the type of the function passed to gdk_pixbuf_save_to_callback(). It is called once for each block of bytes that is "written" by gdk_pixbuf_save_to_callback(). If successful it should return %TRUE. If an error occurs it should set @error and return %FALSE, in which case gdk_pixbuf_save_to_callback() will fail with the same error.