D10415: Fix realDpi function for Mac

[René J.V. Bertin]

Hi Sergio,

>   the old version was giving me some crap, while I was testing non-native resolutions. 

Well, deprecated code is probably not maintained beyond changes required to compile it, so may not function correctly with newer hardware or drivers. Other than that Apple is sometimes surprisingly conservative in keeping deprecate functions and features (then again, this is in *Core*Foundation).

>   I don't have 10.9 at hand, so, it would be good if you could check that part of Linux/pure Qt code. I confirm that it works with 10.11 and 10.12.

Do we agree that my patch is the minimum way of achieving the same thing your patch does? Not that I want to be lazy, but if it is I can already confirm that I have not noticed any issues with using the standard Utils::realDPI function.

Did you verify the actual size at which elements are shown? If so maybe you can your test document and protocol so I can verify this on 10.9 (and maybe have it verified on 10.13 by one of my users)?

> Yes, those things are in progress.

Don't hesitate to borrow from my implementation(s) instead of reinventing the wheel.