Listen to Cocoa delegate windowDidChangeBackingProperties

[shirok1]

This delegate can be used to detect whether the window is moved from a Retina display to a non one (or vice versa). I think this is required for a final perfect solution for GLFW on macOS.

See src/cocoa_window.m for existing delegates.

More info on this:

https://stackoverflow.com/questions/22366117/how-to-detect-when-cocoa-app-switches-from-retina-to-non-retina-screen?rq=3

https://developer.apple.com/library/mac/documentation/GraphicsAnimation/Conceptual/HighResolutionOSX/CapturingScreenContents/CapturingScreenContents.html#//apple_ref/doc/uid/TP40012302-CH10-SW20

[elmindreda]

I'm not sure what benefit this would provide over the current solution that uses viewDidChangeBackingProperties.