matroska: subtitle display duration must be stored in pkt->convergence_duration

Originally committed as revision 15206 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavformat/matroskadec.c b/libavformat/matroskadec.c
index b947a50..437ac37 100644
--- a/libavformat/matroskadec.c
+++ b/libavformat/matroskadec.c
@@ -1583,7 +1583,10 @@
 
                 pkt->pts = timecode;
                 pkt->pos = pos;
-                pkt->duration = duration;
+                if (track->type == MATROSKA_TRACK_TYPE_SUBTITLE)
+                    pkt->convergence_duration = duration;
+                else
+                    pkt->duration = duration;
 
                 dynarray_add(&matroska->packets, &matroska->num_packets, pkt);
             }