avif encoding

[suntong]

First, building with the Dockerfile, which is supposed to support avif encoding, but I'm getting:

/vips/bin/vips copy smart-crop.jpg smart-crop.avif
VipsForeignSave: "smart-crop.avif" is not a known file format

Then, building with the my own Dockerfile, it does support avif encoding, but the encoded avif file is way too big than it should be.

I'll look into the heroku dockerfile the libvips author uses trying to update bimg's. This will be quite a diverge from what it used to do, so @h2non, let me know if you want to take my PR or not.

The reason for such dramatic changes are explained here.