bug#56317: 28.1; image-convert not working on Windows

[Eli Zaretskii <eliz@gnu.org>]

> From: Pascal Quesseveur <pquessev@gmail.com>
> Cc: 56317@debbugs.gnu.org
> Date: Thu, 30 Jun 2022 10:27:27 +0200
> 
> >"EZ" == Eli Zaretskii <eliz@gnu.org> writes:
> 
>   EZ> Thanks.  Please try the patch below:
> 
>   Thanks. Is is working fine.

Thanks for testing.

Lars, WDYT about including this in Emacs 28.2?  If you think it's safe
enough, would you mind testing the patch on GNU/Linux?  I don't have
access to any systems with ImageMagick installed.

(In the final commit, I will make the same change in the ffmpeg
method.)

>   I have 2 other remrks about image-converter:
> 
>   - it would be better to use 'magick convert' instead of 'convert' to
>     avoid ambiguities at least under windows,

I don't understand: the actual method invoked in that case is
image-converter--convert-magick.  The image-converter--convert one (if
that's what you mean) is a generic name (see that cl-defmethod?).  So
what is not right here?

>   - bmp images cannot be processed by an external converter as bmp is
>     listed in image-type-file-name-regexps. Except the bmp images are
>     not displayed by the new method on Windows

Emacs 29 can display BMP images on Windows natively, so that problem
is already taken care of, just not in Emacs 28.