rtpdec: only use RTCP for PTS when synchronizing multiple streams

RTCP timestamps are only necessary to synchronize time between
multiple streams. For a single stream, the RTP packet timestamp
provides more reliable timing. As a result, single-stream RTP
sessions should now have accurate and monotonic PTS.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/rtpdec.c

@@ -424,7 +424,7 @@ static void finalize_packet(RTPDemuxContext *s, AVPacket *pkt, uint32_t timestam
     if (timestamp == RTP_NOTS_VALUE)
         return;
 
-    if (s->last_rtcp_ntp_time != AV_NOPTS_VALUE) {
+    if (s->last_rtcp_ntp_time != AV_NOPTS_VALUE && s->ic->nb_streams > 1) {
         int64_t addend;
         int delta_timestamp;