Added opt-in support for high-dpi mode in OS X when on a retina display (resolves issue #761).

Added a ‘highdpi’ boolean flag to t.window/love.window.setMode (defaults to false.)
When the window is actually in high-dpi mode on a supported display, the graphics width and height and the mouse position are in pixels, rather than ‘window coordinates’.

Added love.window.getPixelScale. Returns the scale factor of the window from user-space points to pixels (e.g. it will be 1 normally, and 2 on a retina display in OS X with high-dpi mode enabled.)

src/scripts/boot.lua

@@ -300,6 +300,7 @@ function love.init()
 			borderless = false,
 			resizable = false,
 			centered = true,
+			highdpi = false,
 		},
 		modules = {
 			event = true,