Ability to skip defining inputs and do it manually via extra args

[YuriMalinov]

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Right now it's required to provide at least one input. I've got situation when:

1. input definition is complex: -f lavfi -i aevalsrc=0
2. it has to be before extra_args because they refer to it (-map, -shortest)

There was no way to do it besides extending FFmpegBuilder and just copying this method commenting out inputs check - and this way it worked well just with all args defined through addExtraArgs.

Maybe I'm missing something in ffmpeg usage - I've tried several args order in command line and it complained about -i going after other args.