GIMP/OSX - improve Retina High DPI support
GIMP/OSX supporting retina but there are some areas which needs improvements. Probably some of them will require gtk/osx changes and some are GIMP related. We will also need report from Linux users to see if issue is OSX specific or not.
Icon scaling
Looks like that SVG icons are rendered with a lower resolution and later upscaled. Retina screenshot:
Code to check - https://gitlab.gnome.org/GNOME/gtk/blob/gtk-2-24/gtk/gtkicontheme.c#L2936
Canvas scaling
Canvas if set to Pixels is double-sized. How to check: create 400x400px image, set 100% size , screenshot canvas, it will be 800x800. Not clear if it is a normal behavior or something needs improvement.
Side note: on Retina images are double-sized in both Preview and Safari, so it is normal. What is not normal that it is not filled with a x2 resolution but just scaled up.
This PR should fix that