Add support for forced subtitles

[defagos]

Description

This PR adds support for forced subtitles.

Changes made

• Filter forced subtitles in returned legible options.
• Apply forced subtitles automatically when meaningful.
• Add stream packaging documentation since proper support for forced subtitles requires proper delivery in streams themselves.
• Update local test delivery for more consistency.
• Make a few unit test timings more tolerant.
• Add Apple Dolby Atmos sample stream.

Checklist

• APIs have been properly documented (if relevant).
• The documentation has been updated (if relevant).
• New unit tests have been written (if relevant).
• The demo has been updated (if relevant).
• The playground has been updated (if relevant).