mplayer: take tracks from first segment if main file is empty

With Matroska ordered chapters, the main file (i.e. the file you're
playing) can be empty, while all video/audio data is in linked files.
Some files don't even contain the track list, only chapter information.
mpv refused to play these, because normally, the main file dictates the
track layout.

Fix this by using the first segment for track data if no part of the
timeline is sourced from the main file.
demux_mkv: don't terminate if there are no clusters

Matroska segment linking allows abusing Matroska files as playlists
without any actual video/audio/sub data, making files without any
clusters still useful for the frontend.
core: matroska: support concatenated segments

Matroska files can contain multiple segments, which are literally
further Matroska files appended to the main file. They can be referenced
by segment linking.

While this is an extraordinarily useless and dumb feature, we support it
for the hell of it.

This is implemented by adding a further demuxer parameter for skipping
segments. When scanning for linked segments, each file is opened
multiple times, until there are no further segments found. Each segment
will have a separate demuxer instance (with a separate file handle
etc.).

It appears the Matroska spec. has an even worse feature for segments:
live streaming can completely reconfigure the stream by starting a new
segment. We won't add support for it, because there are 0 people on this
earth who think Matroska life streaming is a good idea. (As opposed to
serving Matroska/WebM files via HTTP.)
remove tmp_lace_buffer non-sense

Not sure how my mind got 8k, or how this made sense at all.
demux_mkv: support vp9

Note that ffmpeg doesn't provide a decoder by default yet.
matroska: update dead link