Convert selected legacy skin sprites to grayscale

Matching stable.

Closes https://github.com/ppy/osu/issues/9858.

I would have liked to apply this in the taiko transformer itself, but
the limited accessibility of texture uploads, and as such the raw image
data, sort of prevents that...

osu.Game/Skinning/LegacySkin.cs

@@ -58,6 +58,9 @@ protected LegacySkin(SkinInfo skin, IStorageResourceProvider? resources, IResour
         {
         }
 
+        protected override IResourceStore<TextureUpload> CreateTextureLoaderStore(IStorageResourceProvider resources, IResourceStore<byte[]> storage)
+            => new LegacyTextureLoaderStore(base.CreateTextureLoaderStore(resources, storage));
+
         protected override void ParseConfigurationStream(Stream stream)
         {
             base.ParseConfigurationStream(stream);

osu.Game/Skinning/LegacyTextureLoaderStore.cs

@@ -0,0 +1,92 @@
+// 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 System.Collections.Generic;
+using System.IO;
+using System.Threading;
+using System.Threading.Tasks;
+using osu.Framework.Graphics.Textures;
+using osu.Framework.IO.Stores;
+using SixLabors.ImageSharp;
+using SixLabors.ImageSharp.Processing;
+
+namespace osu.Game.Skinning
+{
+    public class LegacyTextureLoaderStore : IResourceStore<TextureUpload>
+    {
+        private readonly IResourceStore<TextureUpload>? wrappedStore;
+
+        public LegacyTextureLoaderStore(IResourceStore<TextureUpload>? wrappedStore)
+        {
+            this.wrappedStore = wrappedStore;
+        }
+
+        public TextureUpload Get(string name)
+        {
+            var textureUpload = wrappedStore?.Get(name);
+
+            if (textureUpload == null)
+                return null!;
+
+            return shouldConvertToGrayscale(name)
+                ? convertToGrayscale(textureUpload)
+                : textureUpload;
+        }
+
+        public Task<TextureUpload> GetAsync(string name, CancellationToken cancellationToken = new CancellationToken())
+        {
+            var textureUpload = wrappedStore?.Get(name);
+
+            if (textureUpload == null)
+                return null!;
+
+            return shouldConvertToGrayscale(name)
+                ? Task.Run(() => convertToGrayscale(textureUpload), cancellationToken)
+                : Task.FromResult(textureUpload);
+        }
+
+        // https://github.com/peppy/osu-stable-reference/blob/013c3010a9d495e3471a9c59518de17006f9ad89/osu!/Graphics/Textures/TextureManager.cs#L91-L96
+        private static readonly string[] grayscale_sprites =
+        {
+            @"taiko-bar-right",
+            @"taikobigcircle",
+            @"taikohitcircle",
+            @"taikohitcircleoverlay"
+        };
+
+        private bool shouldConvertToGrayscale(string name)
+        {
+            foreach (string grayscaleSprite in grayscale_sprites)
+            {
+                // unfortunately at this level of lookup we can encounter `@2x` scale suffixes in the name,
+                // so straight equality cannot be used.
+                if (name.StartsWith(grayscaleSprite, StringComparison.OrdinalIgnoreCase))
+                    return true;
+            }
+
+            return false;
+        }
+
+        private TextureUpload convertToGrayscale(TextureUpload textureUpload)
+        {
+            var image = Image.LoadPixelData(textureUpload.Data.ToArray(), textureUpload.Width, textureUpload.Height);
+
+            // stable uses `0.299 * r + 0.587 * g + 0.114 * b`
+            // (https://github.com/peppy/osu-stable-reference/blob/013c3010a9d495e3471a9c59518de17006f9ad89/osu!/Graphics/Textures/pTexture.cs#L138-L153)
+            // which matches mode BT.601 (https://en.wikipedia.org/wiki/Grayscale#Luma_coding_in_video_systems)
+            image.Mutate(i => i.Grayscale(GrayscaleMode.Bt601));
+
+            return new TextureUpload(image);
+        }
+
+        public Stream? GetStream(string name) => wrappedStore?.GetStream(name);
+
+        public IEnumerable<string> GetAvailableResources() => wrappedStore?.GetAvailableResources() ?? Array.Empty<string>();
+
+        public void Dispose()
+        {
+            wrappedStore?.Dispose();
+        }
+    }
+}