Call func with user_data for each of element'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 func returns 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.
Call func with user_data for each of element'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 func returns 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.