Refactored byte[] array in Octree type

src/ImageSharp/Processing/Processors/Quantization/OctreeFrameQuantizer{TPixel}.cs

@@ -6,6 +6,7 @@ using System.Buffers;
 using System.Collections.Generic;
 using System.Numerics;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 using SixLabors.ImageSharp.Advanced;
 using SixLabors.ImageSharp.Memory;
 using SixLabors.ImageSharp.PixelFormats;
@@ -111,21 +112,6 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
         /// </summary>
         private sealed class Octree
         {
-            /// <summary>
-            /// Mask used when getting the appropriate pixels for a given node.
-            /// </summary>
-            private static readonly byte[] Mask = new byte[]
-            {
-                0b10000000,
-                0b1000000,
-                0b100000,
-                0b10000,
-                0b1000,
-                0b100,
-                0b10,
-                0b1
-            };
-
             /// <summary>
             /// The root of the Octree
             /// </summary>
@@ -162,6 +148,21 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
                 this.previousNode = null;
             }
 
+            /// <summary>
+            /// Gets the mask used when getting the appropriate pixels for a given node.
+            /// </summary>
+            private static ReadOnlySpan<byte> Mask => new byte[]
+            {
+                0b10000000,
+                0b1000000,
+                0b100000,
+                0b10000,
+                0b1000,
+                0b100,
+                0b10,
+                0b1
+            };
+
             /// <summary>
             /// Gets or sets the number of leaves in the tree
             /// </summary>
@@ -513,8 +514,12 @@ namespace SixLabors.ImageSharp.Processing.Processors.Quantization
                 [MethodImpl(InliningOptions.ShortMethod)]
                 private static int GetColorIndex(ref Rgba32 color, int level)
                 {
+                    DebugGuard.MustBeLessThan(level, Mask.Length, nameof(level));
+
                     int shift = 7 - level;
-                    byte mask = Mask[level];
+                    ref byte maskRef = ref MemoryMarshal.GetReference(Mask);
+                    byte mask = Unsafe.Add(ref maskRef, level);
+
                     return ((color.R & mask) >> shift)
                          | ((color.G & mask) >> (shift - 1))
                          | ((color.B & mask) >> (shift - 2));