[[ HiDPI ]] Revised hi-dpi operation to be automatic on desktop:
changed stack device_* methods to logical-coordinate based platform-specific implementations
removed logical -> device coordinate conversions in any platform-agnostic window / screen functions
removed device rects from MCDisplay and added field for display pixel scale
removed MCResUserToDeviceRect / MCResDeviceToUserRect and all usages

OSX - use backing scale of the target context when redrawing - provides automatic hi-dpi support on retina displays
OSX - reimplemented MCScreen::platform_getdisplays() to use Cocoa