Standardise control point search logic in OverlappingScrollAlgorithm

Was using a very local algorithm which I cannot guarantee is correct.
I'd rather it just use the one used everywhere else.

osu.Game/Beatmaps/ControlPoints/IControlPoint.cs

@@ -0,0 +1,13 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Beatmaps.ControlPoints
+{
+    public interface IControlPoint
+    {
+        /// <summary>
+        /// The time at which the control point takes effect.
+        /// </summary>
+        public double Time { get; }
+    }
+}