Switch from aom to svt-av1-psy

[CutestNekoAqua]

Feature request

svt-av1-psy is a (potentially) better encoder for AV1 and Avif offering both a huge speed increase over aom and much better perceptual image quality.

We encountered this issue with sharp thanks to one of our contributors @snaakey who is severely autistic when it comes to Avif images.

Also if this library switches, it could potentially also expose more options for video encoding with avif.

[CutestNekoAqua]

PS: thanks for maintaining a awesome library :)
saying this as most people underestimate the work of library devs 🌷

[snaakey]

Just to clarify SVT-AV1-PSY is a soft fork of SVT-AV1 with overall better perceptual quality, some changed defaults and some additional options that are very useful.

[snaakey]

I just realised that you are using libvips and not libaom directly

[snaakey]

Nontheless exposing all encoder options in this library would be extremely awesome and I suspect that vips defaults to aom if it just uses libheif because that's libheifs default.

[lovell]

A future possible switch to an alternative AV1 encoder as part of the prebuilt binaries is being discussed at lovell/sharp-libvips#97 - I've added a note about svt-av1-psy, thanks for the suggestion - let's continue to track it there.