ffmpeg: use standard duration calculation code in the stream copy case.

Fixes a.mkv non monotonic timestamps

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

ffmpeg.c

@@ -2053,8 +2053,10 @@ static int output_packet(InputStream *ist,
                              ist->st->codec->sample_rate;
             break;
         case AVMEDIA_TYPE_VIDEO:
-            if (ist->st->codec->time_base.num != 0) {
-                int ticks = ist->st->parser ? ist->st->parser->repeat_pict+1 : ist->st->codec->ticks_per_frame;
+            if (pkt->duration) {
+                ist->next_pts += av_rescale_q(pkt->duration, ist->st->time_base, AV_TIME_BASE_Q);
+            } else if(ist->st->codec->time_base.num != 0) {
+                int ticks= ist->st->parser ? ist->st->parser->repeat_pict+1 : ist->st->codec->ticks_per_frame;
                 ist->next_pts += ((int64_t)AV_TIME_BASE *
                                   ist->st->codec->time_base.num * ticks) /
                                   ist->st->codec->time_base.den;