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?

I tried GetDelegateList, but it returned some delegates on whose
calling MagickRelinquishMemory caused a crash during bootstrapping.
There is no crash when calling MagickRelinquishMemory not during
bootstrapping, and I have no idea what is wrong.