Improve playlist alignment when PDT across playlists is inconsistent

[robwalch]

This PR will...

• Handle certain cases where live playlists should not align on PDT.
• Fixes subtitle start offset when adjusted before first segment load.

Why is this Pull Request needed?

Are there any points in the code the reviewer needs to double check?

Resolves issues:

Checklist

• [ ] changes have been done against master branch, and PR does not conflict
• [ ] new unit / functional tests have been added (whenever applicable)
• [ ] API or design changes are documented in API.md