sub: do not apply timeline offset to external subtitles

Now external subtitles essentially use the playback time, instead of
the segment time.

This is more useful when using external subtitles with mkv ordered
chapters. The previous behavior is not necessarily incorrect, and e.g.
makes it easier to use subtitles directly extracted from ordered
chapters segments. But we consider the new behavior more useful.

Also see commit 06e3dc8.