MacGUI: make the preview window HiDPI aware, so that the preview images will be shown with the same pixel count on Retina and non Retina screens (instead of 2x on Retina).

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@5226 b64f7644-9d1e-0410-96f1-a4d463321fa5