rtpbin: RTX packets only forwarded after two are received (probation period)

Submitted by Sebastian Dröge @slomo

Link to original bug (#747919)

Description

Currently an RTX packet is queued up in rtpsource on the receiver until a second one is received... because of the probation period. That's obviously not a good idea for RTX packets, as that one will then be far too late and we don't really have that many packets anyway.

Problem now is that nothing in rtpbin knows about RTX, and somehow we need to get this information to rtpsource so that it does not queue up things.