Add basic background display system.

osu.Game/Graphics/Containers/ParallaxContainer.cs

@@ -0,0 +1,40 @@
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Graphics;
+using osu.Framework.Input;
+using OpenTK;
+
+namespace osu.Game.Graphics.Containers
+{
+    class ParallaxContainer : LargeContainer
+    {
+        public float ParallaxAmount = 0.02f;
+
+        public override bool Contains(Vector2 screenSpacePos) => true;
+
+        private Container content = new LargeContainer()
+        {
+            Anchor = Anchor.Centre,
+            Origin = Anchor.Centre
+        };
+
+        protected override Container AddTarget => content;
+
+        public override void Load()
+        {
+            base.Load();
+            Add(content);
+        }
+
+        protected override bool OnMouseMove(InputState state)
+        {
+            content.Position = (state.Mouse.Position - ActualSize / 2) * ParallaxAmount;
+            return base.OnMouseMove(state);
+        }
+
+        protected override void Update()
+        {
+            base.Update();
+            content.Scale = new Vector2(1 + ParallaxAmount);
+        }
+    }
+}