matroskadec: full seeking support in index-less files

when trying to seek past the last index entry, just parse more clusters to add them to the index, until an index entry match the desired position

Originally committed as revision 14959 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/matroskadec.c

@@ -1657,11 +1657,15 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     if (timestamp < 0)
         timestamp = 0;
 
-    index = av_index_search_timestamp(st, timestamp, flags);
-    if (index < 0)
-        return 0;
+    while ((index = av_index_search_timestamp(st, timestamp, flags)) < 0) {
+        matroska_clear_queue(matroska);
+        if (matroska_parse_cluster(matroska) < 0)
+            break;
+    }
 
     matroska_clear_queue(matroska);
+    if (index < 0)
+        return 0;
 
     url_fseek(s->pb, st->index_entries[index].pos, SEEK_SET);
     matroska->skip_to_keyframe = !(flags & AVSEEK_FLAG_ANY);