P3 - Make sure LPMS works without video

support audio-only streams for transcoding Why? These will just be rejected right now. Since we already don’t support this, low priority, but would be nice to get to.

Right now LPMS will not work when there is no video data. For example, lpms/cmd/transcoding go will panic with TranscoderInvalidVideo error. This is because of the following code in ffmpeg.go:880

if videoMissing {
    // Audio-only segment, fail fast right here as we cannot handle them nicely
    return nil, ErrTranscoderVid
}

When statement above gets commented out, transcoding will crash with SIGSEGV, most likely reference to some kind of video-related pointer that isn't checking against NULL.

Since the idea is to separate video and audio processing at some point during the refactoring process, and since there are no fundamental reasons not to support video-less streams, it could make sense to change LPMS so that it would also work without video.

BTW, to obtain audio-only .ts container one can do something like this:
ffmpeg -i file_that_contains_video_and_audio.format" -map 0:a just_audio.ts