sub: adjust offsets when sub seeking/stepping

In the sub seek code path, there was an arbitrary small offset added to
the pts before the seek. However when seeking backwards, the offset was
an additional subtraction. de6eace6e984be3cd2515e9be6362a0cf04b7457
added this logic 10 years ago and perhaps it made sense then, but the
additional subtraction when seeking backwards causes the subtitle seek
to go too far to the previous subtitle if the durations overlap. This
should always be an addition to work correctly. Additionally, the sub
stepping code path also could use this offset for the same reason
(duration overlaps). However, it is only applicable to sd_ass not
sd_lavc. sd_lavc has step_sub support but on a sample it didn't even
work anyway. Perhaps it only works for certain kinds of subtitles
(patches welcome).

Anyways instead of keeping this offset as a magic number, we can define
it in sd.h which is handy for this. For sd_ass, we add the offset when
sub stepping, and the offset is always added for sub seeking like it was
before. Update the comment to be a little more relevant to what actually
happens today. Fixes #11445.

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