[Mac] Don't block on NextEvent() call
[Mac] Fix key repeat and connect legacy keyboard events
[Mac] Add support for hidpi (retina) displays
[Mac] Width and Height are derived from ClientSize/ClientRectangle

This aligns the Cocoa backend with what we do no Windows and SDL2 on
hidpi displays.
Merge pull request #1 from thefiddler/cocoa

[Mac] Improve Cocoa event handling