Added grayscale app14 support, build fix

src/ImageSharp/Formats/Jpeg/JpegDecoderCore.cs

@@ -510,6 +510,11 @@ namespace SixLabors.ImageSharp.Formats.Jpeg
         /// <returns>The <see cref="JpegColorSpace"/></returns>
         internal static JpegColorSpace DeduceJpegColorSpace(byte componentCount, ref AdobeMarker adobeMarker)
         {
+            if (componentCount == 1)
+            {
+                return JpegColorSpace.Grayscale;
+            }
+
             if (componentCount == 3)
             {
                 if (adobeMarker.ColorTransform == JpegConstants.Adobe.ColorTransformUnknown)

src/ImageSharp/Formats/Tiff/Compression/Decompressors/TiffJpegSpectralConverter{TPixel}.cs

@@ -1,5 +1,5 @@
 // Copyright (c) Six Labors.
-// Licensed under the Apache License, Version 2.0.
+// Licensed under the Six Labors Split License.
 
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder;
 using SixLabors.ImageSharp.Formats.Jpeg.Components.Decoder.ColorConverters;

tests/ImageSharp.Tests/Formats/Jpg/JpegDecoderTests.Internal.cs

@@ -25,6 +25,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Jpg
     public partial class JpegDecoderTests
     {
         [Theory]
+        [InlineData(1, 0, JpegColorSpace.Grayscale)]
         [InlineData(3, JpegConstants.Adobe.ColorTransformUnknown, JpegColorSpace.RGB)]
         [InlineData(3, JpegConstants.Adobe.ColorTransformYCbCr, JpegColorSpace.YCbCr)]
         [InlineData(4, JpegConstants.Adobe.ColorTransformUnknown, JpegColorSpace.Cmyk)]