Support AVIF format

[Cerdic]

AVIF seems is coming fast as a new supported format:

https://caniuse.com/?search=avif
https://jakearchibald.com/2020/avif-has-landed/

In HTML5 markup option we could provide a avif version in a dedicated <source>, keeping the jpg <source> as a fallback for all other browsers.

This would fit cause of:

• the on demand image production, as we don't need to pre-process all the images
• the preview image that counterbalance the lack of progressive loading on avif images

https://docs.fileformat.com/image/avif/

It seems also that avif image format is supported by uptodate ImageMagick lib with libheif :
ImageMagick/ImageMagick#1432

[Cerdic]

maybe see also the lib sharp https://sharp.pixelplumbing.com/