bug#51381: HEIF image format support

[Stefan Kangas]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Yup.  :-)

I finally got around to testing this, with an image from

    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.

I also tried image1.heic from here:

    https://github.com/tigranbs/test-heic-images

with the same result.

Am I doing something wrong?  I have all three of GraphicsMagick, ffmpeg
and ImageMagick installed AFAICT.