bug#33587: [PROPOSED] Default to disabling ImageMagick

[Paul Eggert]

Elias Mårtenson wrote in 
<http://lists.gnu.org/r/emacs-devel/2018-12/msg00186.html> that image 
scaling via Xrender is surprisingly simple. So perhaps an X11 expert 
could investigate doing that for the X Window System, when ImageMagick 
scaling is not available or not used. My impression is that the Xrender 
extension (introduced in 2000) is reasonably popular among X11 servers 
these days.

Scaling on the server could also be faster (e.g., with hardware 
acceleration) and/or more reliable, so quite possibly it'd be better to 
use Xrender to scale even if ImageMagick is available.