muxer: regenerate init.mp4 only when its content changes

[aler9]

No description provided.

[codecov]

Codecov Report

Merging #100 (982032e) into main (66221e4) will increase coverage by 0.10%.
The diff coverage is 60.00%.

@@            Coverage Diff             @@
##             main     #100      +/-   ##
==========================================
+ Coverage   69.22%   69.32%   +0.10%     
==========================================
  Files          42       42              
  Lines        3620     3632      +12     
==========================================
+ Hits         2506     2518      +12     
- Misses        927      928       +1     
+ Partials      187      186       -1     

Files	Coverage Δ
muxer_segment_fmp4.go	72.88% <100.00%> (+0.70%)	⬆️
muxer_segmenter_fmp4.go	68.80% <100.00%> (+0.36%)	⬆️
muxer_server.go	68.36% <100.00%> (+0.73%)	⬆️
muxer_segment_mpegts.go	80.89% <0.00%> (-1.86%)	⬇️
muxer_segment.go	62.50% <0.00%> (-8.93%)	⬇️

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