Fix rounding direction for calculation of AVPacket.duration.

Fixes issue1579

Originally committed as revision 22802 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/utils.c

@@ -886,7 +886,7 @@ static void compute_pkt_fields(AVFormatContext *s, AVStream *st,
     if (pkt->duration == 0) {
         compute_frame_duration(&num, &den, st, pc, pkt);
         if (den && num) {
-            pkt->duration = av_rescale(1, num * (int64_t)st->time_base.den, den * (int64_t)st->time_base.num);
+            pkt->duration = av_rescale_rnd(1, num * (int64_t)st->time_base.den, den * (int64_t)st->time_base.num, AV_ROUND_DOWN);
 
             if(pkt->duration != 0 && s->packet_buffer)
                 update_initial_durations(s, st, pkt);