Add support for right mouse absolute scrolling (when enabled)

Will likely need to be bindable when hooked up to settings and actually used.

osu.Game/Graphics/Containers/OsuScrollContainer.cs

@@ -0,0 +1,72 @@
+using osu.Framework.Graphics.Containers;
+using osu.Framework.Input;
+using OpenTK.Input;
+
+namespace osu.Game.Graphics.Containers
+{
+    class OsuScrollContainer : ScrollContainer
+    {
+        /// <summary>
+        /// Add the ability to seek to an absolute scroll position when the right mouse button is pressed or dragged.
+        /// Uses the value of <see cref="DistanceDecayOnRightMouseScrollbar"/> to smoothly scroll to the dragged location.
+        /// </summary>
+        public bool RightMouseScrollbar = false;
+
+        /// <summary>
+        /// Controls the rate with which the target position is approached when performing a relative drag. Default is 0.02.
+        /// </summary>
+        public double DistanceDecayOnRightMouseScrollbar = 0.02;
+
+        private bool shouldPerformRelativeDrag(InputState state) => RightMouseScrollbar && state.Mouse.IsPressed(MouseButton.Right);
+
+        private void scrollToRelative(float value) => ScrollTo(Clamp((value - Scrollbar.DrawSize[ScrollDim] / 2) / Scrollbar.Size[ScrollDim]), true, DistanceDecayOnRightMouseScrollbar);
+
+        private bool mouseScrollBarDragging;
+
+        protected override bool IsDragging => base.IsDragging || mouseScrollBarDragging;
+
+        protected override bool OnMouseDown(InputState state, MouseDownEventArgs args)
+        {
+            if (shouldPerformRelativeDrag(state))
+            {
+                scrollToRelative(state.Mouse.Position[ScrollDim]);
+                return true;
+            }
+
+            return base.OnMouseDown(state, args);
+        }
+
+        protected override bool OnDrag(InputState state)
+        {
+            if (mouseScrollBarDragging)
+            {
+                scrollToRelative(state.Mouse.Position[ScrollDim]);
+                return true;
+            }
+
+            return base.OnDrag(state);
+        }
+
+        protected override bool OnDragStart(InputState state)
+        {
+            if (shouldPerformRelativeDrag(state))
+            {
+                mouseScrollBarDragging = true;
+                return true;
+            }
+
+            return base.OnDragStart(state);
+        }
+
+        protected override bool OnDragEnd(InputState state)
+        {
+            if (mouseScrollBarDragging)
+            {
+                mouseScrollBarDragging = false;
+                return true;
+            }
+
+            return base.OnDragEnd(state);
+        }
+    }
+}