playaback: use a fixed fMP4 part duration

[aler9]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 65.21739% with 16 lines in your changes are missing coverage. Please review.

Project coverage is 55.76%. Comparing base (0a8e626) to head (58c91fc).
Report is 1 commits behind head on main.

Files	Patch %	Lines
internal/playback/muxer_fmp4.go	72.22%	8 Missing and 2 partials ⚠️
internal/playback/segment_fmp4.go	25.00%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3203      +/-   ##
==========================================
+ Coverage   55.74%   55.76%   +0.01%     
==========================================
  Files         154      154              
  Lines       16917    16941      +24     
==========================================
+ Hits         9431     9447      +16     
- Misses       6741     6748       +7     
- Partials      745      746       +1     

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[github-actions]

This issue is mentioned in release v1.7.0 🚀
Check out the entire changelog by clicking here