demux_mkv: enable automatic index generation by default

Change demux_mkv to behave by default as it did with -idx before. The
index generation code in demux_mkv linearly scans the file up to the
seek timestamp (it doesn't read the whole file up front like some
other demuxers do). Doing that is probably a better default for files
with no index than rejecting the seek request and asking user to
specify -idx.

libmpdemux/demux_mkv.c

@@ -1770,7 +1770,7 @@ static int demux_mkv_open(demuxer_t *demuxer)
         demuxer->audio->id = -2;
     }
 
-    if (s->end_pos == 0 || (mkv_d->indexes == NULL && index_mode < 0))
+    if (s->end_pos == 0)
         demuxer->seekable = 0;
     else {
         demuxer->movi_start = s->start_pos;