rtspsrc: De-dup seek event seqnums to avoid multiple seeks
Seek events are sent upstream on each sink, so if we receive multiple seeks with the same seqnum, we must only perform one seek, not N seeks where N = the number of sinks in the pipeline connected to rtspsrc.
This is the same thing done by demuxers like qtdemux or matrsokademux.
Edited by Nirbheek Chauhan