matroskaenc: base default duration on avg_frame_rate if possible.

Fixed Ticket432
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/matroskaenc.c

@@ -574,7 +574,10 @@ static int mkv_write_tracks(AVFormatContext *s)
         switch (codec->codec_type) {
             case AVMEDIA_TYPE_VIDEO:
                 put_ebml_uint(pb, MATROSKA_ID_TRACKTYPE, MATROSKA_TRACK_TYPE_VIDEO);
-                put_ebml_uint(pb, MATROSKA_ID_TRACKDEFAULTDURATION, av_q2d(codec->time_base)*1E9);
+                if(st->avg_frame_rate.num && st->avg_frame_rate.den && 1.0/av_q2d(st->avg_frame_rate) > av_q2d(codec->time_base))
+                    put_ebml_uint(pb, MATROSKA_ID_TRACKDEFAULTDURATION, 1E9/av_q2d(st->avg_frame_rate));
+                else
+                    put_ebml_uint(pb, MATROSKA_ID_TRACKDEFAULTDURATION, av_q2d(codec->time_base)*1E9);
 
                 if (!native_id &&
                       ff_codec_get_tag(codec_movvideo_tags, codec->codec_id) &&