Added pixel shader tests

tests/ImageSharp.Tests/Processing/Processors/Effects/PixelShaderTest.cs

@@ -0,0 +1,53 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+using System.Numerics;
+
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
+
+using Xunit;
+
+namespace SixLabors.ImageSharp.Tests.Processing.Processors.Effects
+{
+    [GroupOutput("Effects")]
+    public class PixelShaderTest
+    {
+        [Theory]
+        [WithFile(TestImages.Png.CalliphoraPartial, PixelTypes.Rgba32)]
+        public void FullImage<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            provider.RunValidatingProcessorTest(
+                x => x.ApplyPixelShaderProcessor(
+                span =>
+                {
+                    for (int i = 0; i < span.Length; i++)
+                    {
+                        Vector4 v4 = span[i];
+                        float avg = (v4.X + v4.Y + v4.Z) / 3f;
+                        span[i] = new Vector4(avg);
+                    }
+                }),
+                appendPixelTypeToFileName: false);
+        }
+
+        [Theory]
+        [WithFile(TestImages.Png.CalliphoraPartial, PixelTypes.Rgba32)]
+        public void InBox<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            provider.RunRectangleConstrainedValidatingProcessorTest(
+                (x, rect) => x.ApplyPixelShaderProcessor(
+                    span =>
+                    {
+                        for (int i = 0; i < span.Length; i++)
+                        {
+                            Vector4 v4 = span[i];
+                            float avg = (v4.X + v4.Y + v4.Z) / 3f;
+                            span[i] = new Vector4(avg);
+                        }
+                    }, rect));
+        }
+    }
+}