Save functions used by [method@GdkPixbuf.Pixbuf.save_to_callback].
This function 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.
Save functions used by [method@GdkPixbuf.Pixbuf.save_to_callback].
This function 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.