Put ffmpeg custom args after ab-av1 internal args

[stderr-to-devnull]

Currently there is no way of overriding audio encoding because ab-av1 passes its own audio mapping at the end, after the custom args supplied with --enc.

Attempting to encode the audio streams with specific encoders and settings will not work, instead the streams are copied because ab-av1 passed -c:a copy to ffmpeg at the very end of the command syntax, making it impossible to override any audio processing.

Using the following:

--enc c:a:0=libopus --enc b:a:0=256k --enc channel_layout:a:0=5.1 --enc c:a:1=libopus --enc b:a:1=128k

...or the following (attempting to disable audio processing then re-mapping audio streams):

--video-only --enc map=0:a --enc c:a:0=libopus --enc b:a:0=256k --enc channel_layout:a:0=5.1 --enc c:a:1=libopus --enc b:a:1=128k

...will still result in audio streams being copied.

Enabling RUST_LOG='ab_av1=debug', shows that ab-av1 passes -c:a copy AFTER the custom --enc params, nullifying them.

[alexheretic]

The way I use it is with the --acodec arg. It should work as expected using that. However, I think we can probably improve the enc arg logic to handle this style of usage too.

[stderr-to-devnull]

That looks like a global option and only concerning the CODEC. So I can't specify different CODECs for different audio tracks. And I will still have to set bitrate via --enc anyway. All is needed is for ab-av1 to not interfere in this process.

[mr44er]

All is needed is for ab-av1 to not interfere in this process.

--video-only does what it says.
The rest (audio, subs, chapter) I do manually with ffmpeg and mkvmerge.