set avg frame rate in mkv demuxer

Originally committed as revision 23152 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -1390,6 +1390,8 @@ static int matroska_read_header(AVFormatContext *s, AVFormatParameters *ap)
                       255);
             if (st->codec->codec_id != CODEC_ID_H264)
             st->need_parsing = AVSTREAM_PARSE_HEADERS;
+            if (track->default_duration)
+                st->avg_frame_rate = av_d2q(1000000000.0/track->default_duration, INT_MAX);
         } else if (track->type == MATROSKA_TRACK_TYPE_AUDIO) {
             st->codec->codec_type = AVMEDIA_TYPE_AUDIO;
             st->codec->sample_rate = track->audio.out_samplerate;