Add letterbox/screen scaling support

osu.Game/Configuration/ScalingMode.cs

@@ -0,0 +1,12 @@
+// Copyright (c) 2007-2019 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+namespace osu.Game.Configuration
+{
+    public enum ScalingMode
+    {
+        Off,
+        Everything,
+        ExcludeOverlays,
+        Gameplay,
+    }
+}