Paused and backward seeking enhancement

[robwalch]

This PR will...

Use strict tolerance when picking fragments when paused and seeking backward.

Why is this Pull Request needed?

Support seeking back step-by-step without requiring additional configuration that sacrifices performance when seeking forward while playing.

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

This cannot address rendering of buffered media near an unbuffered playhead or rendering subsequently buffered media while paused. I consider these MSE flaws that HLS.js cannot or should not workaround (at least not with this change). For more detail see #6511 (comment).

Does anyone question whether this enhancement should only apply when media is paused (and not when seeking backwards)? When not paused the max frag lookup and buffer hole tolerances allow skipping loading of a whole segment that would only contribute to playback for a small fraction of time. In those cases this could cause more stalls when seeking back near a segment boundary. External pausing and unpausing while seeking could also impact performance in similar ways, so voice you concern if you'd like additional settings to fine-tune this behavior.

Resolves issues:

Resolves #6511 #6331 #4905
Closes #4884

• Seeking to a previous frame, which is the last frame of the previous segment, hangs or seeks incorrectly #6511
• Seeking to the end of a fragment while paused stalls #6331
• Seeking breaks stream with DISCONTINUITY #4905
• Seeking performance issues in Safari (visible in hls.js demo w/ Buffer stalled error) #4884 (can't reproduce)

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