demux_mkv: don't overflow packet queue when doing sub-preroll

Consider the cluster used for prerolling contains an insane amount of
subtitle packets. Then the demuxer packet queue would be full of
subtitle packets, and demux.c would refuse to read any further packets -
including video and audio packets, resulting in EOF. Since everything
involving Matroska and subtitles is 100% insane, this can actually
happen.

Fix this by putting a limit on the number of subtitle packets read by
preroll, and throw away any further packets if the limit is exceeded. If
this happens, the preroll mechanism will stop working, but the player's
operation is unaffected otherwise.