file vidcap doesn't respect user-selected channel count

[MartinPulec]

Originally posted by @TheSashmo in https://github.com/CESNET/UltraGrid/issues/316#issuecomment-1590148014

Quoted:

My command is here: Encode ./UltraGrid-continuous-x86_64.AppImage -s embedded -t switcher -t file:Sintel-1080i59.mp4:loop -t decklink:0 -c libavcodec:encoder=libx264:bitrate=20000k:preset=ultrafast --audio-codec=MP3:sample_rate=48000:bitrate=256k --audio-capture-format channels=2 -m 1316 -P 22816 192.168.99.199 --control-port 10000

[...]

So I set two channels of audio for the encode, but look at the decoder log output, says I have 6 which is what the file actually has, that should not be like that right?:

[MartinPulec]

minimal example of the problem:

$ uv -a channels=2 -s embedded -t file:Sintel-1080i59.mp4 | grep -m 1 peak
[Audio sender] Volume: [0] -24.05/-0.00, [1] -25.04/-0.72, [2] -51.76/-34.41, [3] -37.42/-14.89, [4] -57.24/-37.70, [5] -58.34/-38.94 dBFS RMS/peak