muxer: serve different tracks in different streams

[aler9]

TODO

• check / fix iOS compatibility
• remove zero-duration parts from rendition streams
• fix dumping multiple streams on disk
• fix tests

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 72.73%. Comparing base (a94133b) to head (a34efd7).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
muxer_server.go	50.00%	2 Missing ⚠️
muxer_stream.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
+ Coverage   71.99%   72.73%   +0.74%     
==========================================
  Files          58       58              
  Lines        3881     3881              
==========================================
+ Hits         2794     2823      +29     
+ Misses        898      870      -28     
+ Partials      189      188       -1     

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