Add an AVIF loader

AVIF[1] 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[2].  Image sequences
aren’t implemented yet, nor is HDR or depth channels but those will
require additions to gdk-pixbuf itself.

[1] https://aomediacodec.github.io/av1-avif/
[2] https://github.com/AOMediaCodec/libavif
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