bug#51381: HEIF image format support

[Eli Zaretskii]

> From: Stefan Kangas <stefan@marxist.se>
> Date: Thu, 28 Oct 2021 14:00:32 -0700
> Cc: 51381@debbugs.gnu.org
> 
>     https://github.com/nokiatech/heif_conformance
> 
> on current master:
> 
> 0. curl --output /tmp/C001.heic
> https://raw.githubusercontent.com/nokiatech/heif_conformance/master/conformance_files/C001.heic
> 1. emacs -Q --eval '(setq image-use-external-converter t)' /tmp/C001.heic
> 
> But I get the error message "Unknown image type", and nothing else.

Why do we prefer external-converter for this, instead of having those
images supported natively?  I thought we wanted to move away of
ImageMagick dependencies?