Fix dragging slider bars on nub not performing correctly relative movement

osu.Game/Graphics/UserInterface/OsuSliderBar.cs

@@ -165,6 +165,9 @@ protected override void OnHoverLost(HoverLostEvent e)
             base.OnHoverLost(e);
         }
 
+        protected override bool ShouldHandleAsRelativeDrag(MouseDownEvent e)
+            => Nub.ReceivePositionalInputAt(e.ScreenSpaceMouseDownPosition);
+
         protected override void OnDragEnd(DragEndEvent e)
         {
             updateGlow();