Fix sprite dropdown not showing resources from non-legacy skins during gameplay

osu.Game/Skinning/ISkinTransformer.cs

@@ -0,0 +1,17 @@
+// 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.Skinning
+{
+    /// <summary>
+    /// A skin transformer takes in an <see cref="ISkin"/> and applies transformations to it.
+    /// The most common use case is allowing individual rulesets to add skinnable components without directly coupling to underlying skins.
+    /// </summary>
+    public interface ISkinTransformer : ISkin
+    {
+        /// <summary>
+        /// The original skin that is being transformed.
+        /// </summary>
+        ISkin Skin { get; }
+    }
+}