Implement noise shared logic

src/ImageSharp/Formats/Jxl/Processing/JxlNoiseHelper.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.CompilerServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+internal static class JxlNoiseHelper
+{
+    [MethodImpl(MethodImplOptions.AggressiveInlining)]
+    public static JxlNoiseIndexAndFraction IndexAndFraction(float x)
+    {
+        const int scaleNumerator = JxlNoiseParameters.NoisePoints - 2;
+        const float scale = scaleNumerator / 1.0f;
+
+        float scaledX = MathF.Max(0f, x * scale);
+        float floorX = MathF.Floor(scaledX);
+        float fractionalX = scaledX - floorX;
+
+        if (scaledX >= scaleNumerator + 1)
+        {
+            floorX = scaleNumerator;
+            fractionalX = 1f;
+        }
+
+        return new((int)floorX, fractionalX);
+    }
+}

src/ImageSharp/Formats/Jxl/Processing/JxlNoiseIndexAndFraction.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct JxlNoiseIndexAndFraction(int index, float fraction)
+{
+    public int Index = index;
+
+    public float Fraction = fraction;
+}

src/ImageSharp/Formats/Jxl/Processing/JxlNoiseLevel.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct JxlNoiseLevel(float noiseLevel, float intensity)
+{
+    public float NoiseLevel = noiseLevel;
+
+    public float Intensity = intensity;
+}

src/ImageSharp/Formats/Jxl/Processing/JxlNoiseParameters.cs

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+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing;
+
+internal sealed class JxlNoiseParameters
+{
+    public const int NoisePoints = 8;
+
+    public float[] Lookup { get; set; } = new float[NoisePoints];
+
+    public bool ContainsAny => this.Lookup.Any(x => MathF.Abs(x) > 1e-3f);
+
+    public void Clear() => Array.Fill(this.Lookup, 0f);
+}