user_data for each of this's sink pads.
func will be called exactly once for each sink pad that exists at the time of this call, unless one of the calls to
false in which case we will stop iterating pads and return early. If new sink pads are added or sink pads are
removed while the sink pads are being iterated, this will not be taken into account until next time this function is used.
a Element to iterate sink pads of
function to call for each sink pad
user data passed to