In order to sync RTP streams that get their initial RTCP timestamp at
different times, propagate the NTP timestamp of the first RTCP packet
to all other streams.

This makes the timestamps of returned packets start at (near) zero instead
of at any random offset.


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22917 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b