Added initial version of the pixel shader delegate and processors

src/ImageSharp/Processing/Delegates/PixelShader.cs

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+using System;
+using System.Numerics;
+
+namespace SixLabors.ImageSharp.Processing.Delegates
+{
+    /// <summary>
+    /// A <see langword="delegate"/> representing a user defined pixel shader.
+    /// </summary>
+    /// <param name="span">The target row of <see cref="Vector4"/> pixels to process.</param>
+    /// <remarks>The <see cref="Vector4.X"/>, <see cref="Vector4.Y"/>, <see cref="Vector4.Z"/>, and <see cref="Vector4.W"/> fields map the RGBA channels respectively.</remarks>
+    public delegate void PixelShader(Span<Vector4> span);
+}

src/ImageSharp/Processing/Processors/DelegatePixelShaderProcessor.cs

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+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Delegates;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// Applies a user defined pixel shader effect through a given delegate.
+    /// </summary>
+    public sealed class DelegatePixelShaderProcessor : IImageProcessor
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DelegatePixelShaderProcessor"/> class.
+        /// </summary>
+        /// <param name="pixelShader">
+        /// The user defined pixel shader to use to modify images.
+        /// </param>
+        public DelegatePixelShaderProcessor(PixelShader pixelShader)
+        {
+            this.PixelShader = pixelShader;
+        }
+
+        /// <summary>
+        /// Gets the user defined pixel shader.
+        /// </summary>
+        public PixelShader PixelShader { get; }
+
+        /// <inheritdoc />
+        public IImageProcessor<TPixel> CreatePixelSpecificProcessor<TPixel>(Image<TPixel> source, Rectangle sourceRectangle)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            return new DelegatePixelShaderProcessor<TPixel>(this, source, sourceRectangle);
+        }
+    }
+}

src/ImageSharp/Processing/Processors/DelegatePixelShaderProcessor{TPixel}.cs

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+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System;
+using System.Numerics;
+
+using SixLabors.ImageSharp.Advanced;
+using SixLabors.ImageSharp.Advanced.ParallelUtils;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing.Delegates;
+using SixLabors.Primitives;
+
+namespace SixLabors.ImageSharp.Processing.Processors
+{
+    /// <summary>
+    /// Performs simple binary threshold filtering against an image.
+    /// </summary>
+    /// <typeparam name="TPixel">The pixel format.</typeparam>
+    internal class DelegatePixelShaderProcessor<TPixel> : ImageProcessor<TPixel>
+        where TPixel : struct, IPixel<TPixel>
+    {
+        /// <summary>
+        /// The user defined pixel shader.
+        /// </summary>
+        private readonly PixelShader pixelShader;
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="DelegatePixelShaderProcessor{TPixel}"/> class.
+        /// </summary>
+        /// <param name="definition">The <see cref="DelegatePixelShaderProcessor"/> defining the processor parameters.</param>
+        /// <param name="source">The source <see cref="Image{TPixel}"/> for the current processor instance.</param>
+        /// <param name="sourceRectangle">The source area to process for the current processor instance.</param>
+        public DelegatePixelShaderProcessor(DelegatePixelShaderProcessor definition, Image<TPixel> source, Rectangle sourceRectangle)
+            : base(source, sourceRectangle)
+        {
+            this.pixelShader = definition.PixelShader;
+        }
+
+        /// <inheritdoc/>
+        protected override void OnFrameApply(ImageFrame<TPixel> source)
+        {
+            var interest = Rectangle.Intersect(this.SourceRectangle, source.Bounds());
+            int startX = interest.X;
+            PixelShader pixelShader = this.pixelShader;
+
+            ParallelHelper.IterateRowsWithTempBuffer<Vector4>(
+                interest,
+                this.Configuration,
+                (rows, vectorBuffer) =>
+                {
+                    for (int y = rows.Min; y < rows.Max; y++)
+                    {
+                        Span<Vector4> vectorSpan = vectorBuffer.Span;
+                        int length = vectorSpan.Length;
+                        Span<TPixel> rowSpan = source.GetPixelRowSpan(y).Slice(startX, length);
+                        PixelOperations<TPixel>.Instance.ToVector4(this.Configuration, rowSpan, vectorSpan);
+
+                        // Run the user defined pixel shader on the current row of pixels
+                        pixelShader(vectorSpan);
+
+                        PixelOperations<TPixel>.Instance.FromVector4Destructive(this.Configuration, vectorSpan, rowSpan);
+                    }
+                });
+        }
+    }
+}