* lisp/files.el (auto-mode-alist): Add ".avif" to the list of
image file types supported by 'image-convert'.
Remove duplicate entry for ".webp" that was added at the top
in the list of natively supported image types.
* lisp/image.el (imagemagick-enabled-types): Add AVIF and
WEBP (the latest for the case when native libs are not installed).
* lisp/image/image-converter.el (image-converter--converters):
Add "-layers merge" to the imagemagick convert command line.
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