demux_mkv: better behavior/warnings on partial files/unseekable streams

demux_mkv may seek to the end of the file to read certain headers (which
should probably be called "footers", but in theory they are just headers
that have been placed at the end of the file unfortunately).

This commit changes behavior not to seek if the stream is not marked as
seekable. Before this, it only checked whether the stream size was
unknown (end negative). In practice it doesn't make much of a
difference, since seekable usually equals known stream size.

Also improve the wording, and distinguish between actual incomplete
files, and unseekable ones.

0 files changed, 0 insertions, 0 deletions