bug#37972: Emacs cannot deal with .webp suffix

[Juri Linkov]

>> $ file *
>> X.webp: RIFF (little-endian) data, Web/P image...
>>
>> $ emacs X.webp #binary mess
>> $ ln X.webp X.gif
>> $ emacs X.gif #works fine
>>
>> It doesn't matter what the suffix is: jpg, gif all work the same. Just
>> webp is not recognized yet.

Renaming .webp to .gif displays it, but also spews into stderr:

  Decoded /tmp/magick-6240. Dimensions: 320 x 214 . Format: lossy. Now saving...
  Saved file /tmp/magick-6240

Looks like debug info, should such fprintf be disabled?

Anyway, maybe these messages are less relevant since now we have
image-converter:

> Emacs 27 can display these image formats if you have
>
> (setq image-use-external-converter t)
>
> But this bug report reminded me that we had agreed on defaulting these
> exotic image formats to `image-mode', so I've now done that (as well as
> fixing the UX of being unable to display images slightly).

Thanks.  I tried now to enable image-use-external-converter and visit
a webp image, but it signals an error:

  Cannot display image: (Searching for program No such file or directory gm)

I don't know what this 'gm' is, never used it.  I have only 'convert'
installed.  Is it supported by image-converter?