Add Ycck and CMYK conversion

These should be correct. I couldn't figure out how to test the values though as I can't remember what format each component array is in. Pretty sure in CMYK it's actually Ycc + inverted K.

src/ImageSharp/Formats/Jpeg/Common/Decoder/JpegColorConverter.FromCmyk.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Numerics;
+
+namespace SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder
+{
+    internal abstract partial class JpegColorConverter
+    {
+        private class FromCmyk : JpegColorConverter
+        {
+            public FromCmyk()
+                : base(JpegColorSpace.Cmyk)
+            {
+            }
+
+            public override void ConvertToRGBA(ComponentValues values, Span<Vector4> result)
+            {
+                // TODO: We can optimize a lot here with Vector<float> and SRCS.Unsafe()!
+                ReadOnlySpan<float> cVals = values.Component0;
+                ReadOnlySpan<float> mVals = values.Component1;
+                ReadOnlySpan<float> yVals = values.Component2;
+                ReadOnlySpan<float> kVals = values.Component3;
+
+                var v = new Vector4(0, 0, 0, 1F);
+
+                var scale = new Vector4(1 / 255F, 1 / 255F, 1 / 255F, 1F);
+
+                for (int i = 0; i < result.Length; i++)
+                {
+                    float c = cVals[i];
+                    float m = mVals[i];
+                    float y = yVals[i];
+                    float k = kVals[i] / 255F;
+
+                    v.X = c * k;
+                    v.Y = m * k;
+                    v.Z = y * k;
+                    v.W = 1F;
+
+                    v *= scale;
+
+                    result[i] = v;
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Jpeg/Common/Decoder/JpegColorConverter.FromYccK.cs

@@ -0,0 +1,46 @@
+using System;
+using System.Numerics;
+
+namespace SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder
+{
+    internal abstract partial class JpegColorConverter
+    {
+        private class FromYccK : JpegColorConverter
+        {
+            public FromYccK()
+                : base(JpegColorSpace.Ycck)
+            {
+            }
+
+            public override void ConvertToRGBA(ComponentValues values, Span<Vector4> result)
+            {
+                // TODO: We can optimize a lot here with Vector<float> and SRCS.Unsafe()!
+                ReadOnlySpan<float> yVals = values.Component0;
+                ReadOnlySpan<float> cbVals = values.Component1;
+                ReadOnlySpan<float> crVals = values.Component2;
+                ReadOnlySpan<float> kVals = values.Component3;
+
+                var v = new Vector4(0, 0, 0, 1F);
+
+                var scale = new Vector4(1 / 255F, 1 / 255F, 1 / 255F, 1F);
+
+                for (int i = 0; i < result.Length; i++)
+                {
+                    float y = yVals[i];
+                    float cb = cbVals[i] - 128F;
+                    float cr = crVals[i] - 128F;
+                    float k = kVals[i] / 255F;
+
+                    v.X = (255F - MathF.Round(y + (1.402F * cr), MidpointRounding.AwayFromZero)) * k;
+                    v.Y = (255F - MathF.Round(y - (0.344136F * cb) - (0.714136F * cr), MidpointRounding.AwayFromZero)) * k;
+                    v.Z = (255F - MathF.Round(y + (1.772F * cb), MidpointRounding.AwayFromZero)) * k;
+                    v.W = 1F;
+
+                    v *= scale;
+
+                    result[i] = v;
+                }
+            }
+        }
+    }
+}

src/ImageSharp/Formats/Jpeg/Common/Decoder/JpegColorConverter.cs

@@ -9,7 +9,7 @@ namespace SixLabors.ImageSharp.Formats.Jpeg.Common.Decoder
 {
     internal abstract partial class JpegColorConverter
     {
-        private static readonly JpegColorConverter[] Converters = { new FromYCbCr(), };
+        private static readonly JpegColorConverter[] Converters = { new FromYCbCr(), new FromYccK(), new FromCmyk() };
 
         protected JpegColorConverter(JpegColorSpace colorSpace)
         {