muxer: support multiple audio tracks (bluenviron/mediamtx#2728)

[aler9]

TODO

• fix muxer: serve different tracks in different streams #180
• check / fix iOS compatibility
• audio-only streams

[codecov]

Codecov Report

Attention: Patch coverage is 79.46429% with 23 lines in your changes missing coverage. Please review.

Project coverage is 72.97%. Comparing base (56d25b4) to head (1e57b50).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
muxer.go	71.95%	17 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #181      +/-   ##
==========================================
+ Coverage   72.76%   72.97%   +0.21%     
==========================================
  Files          58       58              
  Lines        3881     3915      +34     
==========================================
+ Hits         2824     2857      +33     
+ Misses        869      867       -2     
- Partials      188      191       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.