ad_lavc: allow skip samples amount to be larger than 1 packet

Apparently we actually need this. At least the following commit would
break without this.
demux_mkv: implement audio skipping/trimming

This mechanism was introduced for Opus, and allows correct skipping of
"preroll" data, as well as discarding trailing audio if the file's
length isn't a multiple of the audio frame size.

Not sure how to handle seeking. I don't understand the purpose of the
SeekPreRoll element.

This was tested with correctness_trimming_nobeeps.opus, remuxed to mka
with mkvmerge v7.2.0. It seems to be correct, although the reported file
duration is incorrect (maybe a mkvmerge issue).
demux_mkv: remove minor code duplication
demux: don't consider stream EOF an underrun

In this case, we didn't find any new packets for this stream, even
though we've read ahead as much as possible. (If reading ahead in this
case, the "Too many packets in the demuxer packet queues" error is
normally printed.)

If we do consider this an underrun, handle_pause_on_low_cache() will
pause and show the "buffering" state, which is not useful.

Could also happen on very bad interleaving.
demux: fix PTS comparison

This was relying on the fact that timestamps will always be numerically
larger than MP_NOPTS_VALUE, but the trick didn't actually work for
MP_PTS_MIN. Be a bit more sincere, and don't rely on this anymore. This
fixes the comparison, and avoids the readahead amount displaying as
"???" in some situations (since one of the values was NOPTS).
demux: don't account known range for streams that are EOF

This influences the demuxer readahead display. If a stream has reached
EOF, we want to ignore it for the purpose of this calculation.

Note that if a stream contains no packets, it still should cause the
value 0s to be displayed (unless it's EOF), because that's just the
actual situation.