avformat/mux: 2 subtitle packets could have the same DTS

Fixes Ticket3514

See: ETSI EN 300 743 V1.3.1 (2006-11)
"In summary, all of the segments of a single display set shall be carried in one (or more) PES packets that have the same
 PTS value."
with PTS = DTS and remuxing of such a stream it is to be expected that sometimes
multiple packets would have the same DTS
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mux.c

@@ -490,6 +490,7 @@ static int compute_pkt_fields2(AVFormatContext *s, AVStream *st, AVPacket *pkt)
 
     if (st->cur_dts && st->cur_dts != AV_NOPTS_VALUE &&
         ((!(s->oformat->flags & AVFMT_TS_NONSTRICT) &&
+          st->codec->codec_type != AVMEDIA_TYPE_SUBTITLE &&
           st->cur_dts >= pkt->dts) || st->cur_dts > pkt->dts)) {
         av_log(s, AV_LOG_ERROR,
                "Application provided invalid, non monotonically increasing dts to muxer in stream %d: %s >= %s\n",