Merge pull request #24 from jeremycw/osx-retina

Fix mouse coordinates on OS X retina displays

platform/osx/os_osx.mm

@@ -378,8 +378,8 @@ static int button_mask=0;
 	prev_mouse_y=mouse_y;
 	const NSRect contentRect = [OS_OSX::singleton->window_view frame];
 	const NSPoint p = [event locationInWindow];
-	mouse_x = p.x;
-	mouse_y = contentRect.size.height - p.y;
+	mouse_x = p.x * [[event window] backingScaleFactor];
+	mouse_y = (contentRect.size.height - p.y) * [[event window] backingScaleFactor];
 	ev.mouse_motion.x=mouse_x;
 	ev.mouse_motion.y=mouse_y;
 	ev.mouse_motion.global_x=mouse_x;