formats/fmp4: improve unmarshaler performance

[aler9]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 63.48837% with 157 lines in your changes are missing coverage. Please review.

Project coverage is 84.29%. Comparing base (f276ff6) to head (3706cb2).

Files	Patch %	Lines
pkg/formats/fmp4/init.go	60.25%	85 Missing and 39 partials ⚠️
pkg/formats/fmp4/parts.go	72.03%	22 Missing and 11 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #112      +/-   ##
==========================================
- Coverage   84.73%   84.29%   -0.44%     
==========================================
  Files          72       72              
  Lines        5619     5635      +16     
==========================================
- Hits         4761     4750      -11     
- Misses        639      657      +18     
- Partials      219      228       +9     

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