options: add secondary-sub-delay

Add --secondary-sub-delay option and decouple --sub-delay from secondary
subtitles. This produces desirable behavior in most cases as secondary
and primary subtitles tracks tend to be timed independently of one
another.

This feature is implemented by turning the sub_delay field in
mp_subtitle_opts into an array of 2 floats. From here the track index is
either passed around or derived when sub_delay is needed. There are some
cases in dec_sub.c where it is possible for dec_sub.order (equivalent to
track index) to be -1. In these cases, sub_delay is inferred as 0.