command: use playback time as reference for sub_seek

update_subtitle() already uees playback_pts to make subtitles work
better in no-audio mode. Using get_current_time() usually gets
playback_pts, but also has the advantage that it will use the seek
target time during seeks. This will result in multiple sub_seek commands
doing the right thing (at least as long as they're far enough apart so
that seeking is actually initiated when the second command is run).
demux_mkv: index all packets

Instead of indexing only 1 packet per cluster (which is enough for
working seeking), add every packet to the index.

Since on seek, we go through every single index entry, this probably
makes seeking slower. On the other hand, this code is used for files
without index only (e.g. incomplete files), so it probably doesn't
matter much.

Preparation for the following commits.
demux_mkv: read CueRelativePosition/CueDuration elements

Nothing is done with them yet. This is preparation for the following
commit.

CueRelativePosition isn't even saved anywhere, because I don't intend to
use it. (Too messy for no gain.)
demux_mkv: apply subtitle preroll only if needed, based on cue index

The demuxer has a hack to seek to the cluster before the target cluster
in order to "catch" subtitle lines that start before the seek target,
but overlap with the video after the seek target.

Avoid this hack if the cue index indicates that there are no overlapping
subtitle packets that can be caught by seeking to the previous cluster.
demux_mkv: for subtitle preroll, consider all clusters

This considered only index entries that were for the same track ID as
the track used for seeking. This doesn't make much sense for preroll;
it'll just possibly skip clusters, and select an earlier cluster.

One possible negative side-effect is that the preroll might be too tight
now, and miss subtitle packets more often.