Change OffsetScrollPosition to take a double

In line with other changes, this should have been switched to `double`.
Also making `virtual` for usage within the beatmap carousel (I need to
extend what this function does for this usage).

Author: peppy

osu.Framework/Graphics/Containers/ScrollContainer.cs

@@ -418,7 +418,7 @@ protected override bool OnScroll(ScrollEvent e)
         /// Immediately offsets the current and target scroll position.
         /// </summary>
         /// <param name="offset">The scroll offset.</param>
-        public void OffsetScrollPosition(float offset)
+        public virtual void OffsetScrollPosition(double offset)
         {
             Target += offset;
             Current += offset;