Fixed wrong image size on retina with external monitor

[leoMehlig]

There was a bug were the icon were the wrong size, because it used the
build-in monitor’s scaleFactor to draw but the mainScreen was the
external monitor, resulting in doubled sized icons

[stringcode86]

Hi, first of all thank you very much for the bug fix and pull request. I have looked over the code and I think I know what you have in mind. I remember I had a lot of headache with getting the image scale right. I am missing something, or API is bad at dealing with it. As I understand it you have non retina external screen with macbook pro retina screen. I have all confidence that it works as expected on your setup now. Thing is I will definitely not have time to test it out till this weekend at best. Likely won't get to till next weekend. With that said, are you sure that it behaves correctly on retina screens ? If you unplug your screen, are the images still correct size ? If you are 100% confident I'll merge it in. Otherwise I will merge it once I got a chance to test it. Thanks again.

[leoMehlig]

I'd definitely wait and test this before merging, since there are so many things to be considered. I think the API is not really consistent and I hope that this method is the right one to use. There are also some other options on SO, but this was the only one which has been working for my setup.
No hurry and thank you for this script. I integrated it with all my apps and even with fastlane for easier beta icon rendering. 👍