Enable hires retina support for the GTK2/OSX build

This patch adds support for HiDPI display in OSX/Mojave. To demonstrate difference screenshot is provided below:

First image is 100x100 png file with 50% zoom without patch, and second one - with patch.

@Jehan would be great if you can review.

P.S. I think it should not be added to master as [hopefully] this will be handled by GTK3 itself, and if not - GTK3 should be patched.