avformat/matroskaenc: use r_frame_rate as fallback to set a track's DefaultDuration

Signed-off-by: NJames Almer <jamrial@gmail.com>

libavformat/matroskaenc.c

@@ -1311,6 +1311,9 @@ static int mkv_write_track(AVFormatContext *s, MatroskaMuxContext *mkv,
         if(   st->avg_frame_rate.num > 0 && st->avg_frame_rate.den > 0
            && av_cmp_q(av_inv_q(st->avg_frame_rate), st->time_base) > 0)
             put_ebml_uint(pb, MATROSKA_ID_TRACKDEFAULTDURATION, 1000000000LL * st->avg_frame_rate.den / st->avg_frame_rate.num);
+        else if(   st->r_frame_rate.num > 0 && st->r_frame_rate.den > 0
+                && av_cmp_q(av_inv_q(st->r_frame_rate), st->time_base) > 0)
+            put_ebml_uint(pb, MATROSKA_ID_TRACKDEFAULTDURATION, 1000000000LL * st->r_frame_rate.den / st->r_frame_rate.num);
 
         if (!native_id &&
             ff_codec_get_tag(ff_codec_movvideo_tags, par->codec_id) &&