Use AVFormatContext.start_time_realtime in the RTP muxer

Originally committed as revision 22543 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/rtpenc.c

@@ -86,6 +86,10 @@ static int rtp_write_header(AVFormatContext *s1)
     s->ssrc = 0; /* FIXME: was random(), what should this be? */
     s->first_packet = 1;
     s->first_rtcp_ntp_time = ff_ntp_time();
+    if (s1->start_time_realtime)
+        /* Round the NTP time to whole milliseconds. */
+        s->first_rtcp_ntp_time = (s1->start_time_realtime / 1000) * 1000 +
+                                 NTP_OFFSET_US;
 
     max_packet_size = url_fget_max_packet_size(s1->pb);
     if (max_packet_size <= 12)