preview-mp4 encoding profile discards channel layout (which the waveform operation then can't handle)

[JulianKniephoff]

Chucking this MKV with 5.1 sound (which is properly recognized by ffprobe) into Opencast generates an MP4 with unknown channel layout from the preview-mp4 encoding profile. (ffprobe just shows "6 channels")

This is weird in and of itself, but it also in turn trips up the waveform operation, which apparently can't handle files with unknown channel layouts.

An easy workaround would be to just downmux the preview video to stereo or something like that. We can of course do that in our default config (for the waveform service and/or the encoding profiles), but to provide a generic solution I think we would have to teach encoding profile and the waveform service about audio channels ... 🤔

Or we fix ffmpeg bc I think both of those behaviors (discarding the layout and needing it for showwavepic) are weird. 👌