demux_mkv: use unique IDs for cover art pseudo-tracks

Might fix behavior with mkv files that use ordered chapters and have
cover art tags. In my opinion, this should actually have worked (because
cover art pseudo-tracks are strictly appended), but I don't have a
sample file to test at hand.

demux/demux_mkv.c

@@ -1133,6 +1133,7 @@ static void add_coverart(struct demuxer *demuxer)
         struct sh_stream *sh = new_sh_stream(demuxer, STREAM_VIDEO);
         if (!sh)
             break;
+        sh->demuxer_id = -1 - sh->index; // don't clash with mkv IDs
         sh->codec = codec;
         sh->attached_picture = new_demux_packet_from(att->data, att->data_size);
         if (sh->attached_picture) {