bug#33282: ImageMagick delegates

[Juri Linkov]

WebP is an image format that is gaining popularity,
but currently can't be displayed by image-mode.

It's supported by ImageMagick after installing

  sudo apt-get install webp

The command `identify -list delegate` confirms that:

  webp => "dwebp' -pam '%i' -o '%o"

It's possible to work around its missing support by adding to ~/.emacs:

  (add-to-list 'imagemagick-enabled-types 'WEBP)

  (advice-add 'imagemagick-types :around
              (lambda (orig-fun &rest args)
                (append (apply orig-fun args) '(WEBP))))

  (imagemagick-register-types)

So the question is: should `imagemagick-types' return
the delegate list in addition to the current list
of non-delegate image types?