Fix capped times delivered by a progress tracker at the live edge

[defagos]

Description

This PR fixes incorrectly capped times delivered by a ProgressTracker at the live edge of a DVR livestream.

Changes made

• Revisit implementation to store uncapped internal progress.
• Always expose progress in 0...1 publicly, but use internal uncapped progress for time construction.

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).