Add a legacy element to help with texture fallbacks

osu.Game.Rulesets.Mania/Skinning/LegacyManiaColumnElement.cs

@@ -0,0 +1,41 @@
+// 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.
+
+using System;
+using JetBrains.Annotations;
+using osu.Framework.Allocation;
+using osu.Framework.Graphics.Containers;
+using osu.Game.Rulesets.Mania.UI;
+
+namespace osu.Game.Rulesets.Mania.Skinning
+{
+    /// <summary>
+    /// A <see cref="CompositeDrawable"/> which is placed somewhere within a <see cref="Column"/>.
+    /// </summary>
+    public class LegacyManiaColumnElement : CompositeDrawable
+    {
+        [Resolved(CanBeNull = true)]
+        [CanBeNull]
+        protected ManiaStage Stage { get; private set; }
+
+        [Resolved]
+        protected Column Column { get; private set; }
+
+        /// <summary>
+        /// The column index to use for texture lookups, in the case of no user-provided configuration.
+        /// </summary>
+        protected int FallbackColumnIndex { get; private set; }
+
+        [BackgroundDependencyLoader]
+        private void load()
+        {
+            if (Stage == null)
+                FallbackColumnIndex = Column.Index % 2 + 1;
+            else
+            {
+                int dist = Math.Min(Column.Index, Stage.Columns.Count - Column.Index - 1);
+                FallbackColumnIndex = dist % 2 + 1;
+            }
+        }
+    }
+}