Skip encoding of file based on existing "acceptable" codec

[stephenjust]

When setting up a transcoding pipeline, sometimes the existing media format might be "acceptable". For example, in a library with vp9, h265, h264, av1 video files, I might only want to transcode files that were in av1 instead of everything that was not the target format.

As an exaple use-case, Plex can handle many media formats, but not av1. I only want to automatically transcode files that are in av1, so that everything is in a format that Plex can read, even if that is spread across many formats.

I did prototype a solution to this as a separate input filter stage, but the challenge is that if you want the same to apply to audio codecs as well, there's no way to indicate that only the video streams should be consumed by tasks.

Either each transcoding plugin would need to implement this logic, or plugins need to be able to selectively inhibit later plugins from running.