Deal with high DPI for Retina on Apple Macs

[davidryan59]

Currently Canvas is set up for standard DPI (pixel = 1)

Apple Mac Retina has pixel = 0.5

Make canvas display nicely on Mac, with compatibility across all browsers

See:

https://coderwall.com/p/vmkk6a/how-to-make-the-canvas-not-look-like-crap-on-retina

https://developer.apple.com/library/archive/documentation/AudioVideo/Conceptual/HTML-canvas-guide/SettingUptheCanvas/SettingUptheCanvas.html