ffmpeg: enable libvmaf

[barrbrain]

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The VMAF models are available since #88931 was merged, so we may enable libvmaf and replace the default model path. This is motivated by tools like Av1an which use the VMAF filter to evaluate and target video quality when encoding. Note that this does not expand the dependency tree of the ffmpeg formula as libvmaf is already a transitive dependency via aom.

[carlocab]

I'm going to cherry pick your commit onto #88821 so that we only have to rev bump ffmpeg once.

[carlocab]

This doesn't seem to be fixed on FFmpeg master. Has this been reported upstream?

[barrbrain]

I will consult with the ffmpeg developers on whether they intend to make this configurable.
My understanding is that it is not practical to autodetect the model path, so this is left to packagers to resolve.

[barrbrain]

The related issues have been reported upstream but there appears not to be any related patches in the past year.
https://trac.ffmpeg.org/ticket/9094
https://trac.ffmpeg.org/ticket/9047
Netflix/vmaf#753
Netflix/vmaf#324

Other packagers have opted to replace the path at build time. I asked in #ffmpeg-devel and and they agreed that replacing the patch in the Homebrew formula is a reasonable solution for the time being.

[carlocab]

Merged in b8e351f. Thanks @barrbrain!