Add an AVIF loader

AVIF is an image format based on the patent-free AV1 video codec. It supports either lossy or lossless images, with or without an alpha channel, still pictures or image sequences, and can even contain HDR metadata. Its extension is .avif, its MIME type is image/avif, and it is overall an ISOBMFF file (usually .mp4).

This loader allows this format to be loaded anywhere gdk-pixbuf is used, as long as it has been compiled against libavif. Image sequences aren’t implemented yet, nor is HDR or depth channels but those will require additions to gdk-pixbuf itself.

Here is how it looks when loading an avif file in Inkscape: