bug#51381: HEIF image format support

[Stefan Kangas]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> It probably autodetects GraphicsMagick?  The logic doesn't look for
> support per image format, but just uses one of the three.
>
> Try setting `image-converter' to `imagemagick' and restart Emacs.

Aha!  Yup, that works.

I have three questions:

A) If e.g. GraphicsMagick fails, shouldn't we try falling back to the
   other converters?

B) Could the above error message be better?  For example, could it say
   which image converter(s) we tried?

C) Shouldn't `image-use-external-converter' be t by default?