fix: use secondary delay for secondary subtitles

[kuator]

Problem

mpv has separate timing settings for primary and secondary subtitles:

• sub-delay for the primary track
• secondary-sub-delay for the secondary track

Mpvacious currently uses sub-delay when calculating the timing of both
tracks. When the two settings differ, the secondary subtitle is read at the
wrong time.

Example

Suppose the primary subtitle delay is 2 seconds and the secondary subtitle
delay is 1 second. Mpvacious currently shifts both tracks by 2 seconds, so the
secondary subtitle is off by 1 second.

After this change, the primary track is shifted by 2 seconds and the secondary
track by 1 second, matching what mpv displays.

Change

Use secondary-sub-delay when reading secondary subtitle timestamps. Primary
subtitles continue to use sub-delay.

Tests

Added a test where the primary and secondary delays are different. The full
test suite passes with luajit tests/run.lua.

[kuator]

#177

[tatsumoto-ren]

Thanks!