user_data for each of this's source pads.
func will be called exactly once for each source 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 source pads are added or source pads are
removed while the source pads are being iterated, this will not be taken into account until next time this function is used.
a Element to iterate source pads of
function to call for each source pad
user data passed to