Add JPEG parser

src/ImageSharp/Formats/Jxl/IO/Jpeg/JpegDebug.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Diagnostics;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Jpeg;
+
+internal static class JpegDebug
+{
+    [Conditional("DEBUG")]
+    public static void LogWarning(string message) =>
+        // TODO: do we allow Debug.WriteLine in this codebase?
+        Debug.WriteLine($"⚠️ JPEG XL [JPEG parser]: {message}");
+}

src/ImageSharp/Formats/Jxl/IO/Jpeg/JpegHuffmanDecoder.cs

@@ -8,8 +8,8 @@ namespace SixLabors.ImageSharp.Formats.Jxl.IO.Jpeg;
 /// </summary>
 internal static class JpegHuffmanDecoder
 {
-    private const int RootTableBits = 8;
+    public const int RootTableBits = 8;
-    private const int LookupSize = 8;
+    public const int LookupSize = 8;
 
     private static int NextTableBitSize(Span<int> count, int length)
     {

src/ImageSharp/Formats/Jxl/IO/Jpeg/JpegReadMode.cs

@@ -0,0 +1,13 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO.Jpeg;
+
+internal enum JpegReadMode : byte
+{
+    ReadHeader,
+
+    ReadTables,
+
+    ReadEverything
+}

src/ImageSharp/Formats/Jxl/IO/Metadata/JxlExtraChannelInfo.cs

@@ -23,5 +23,17 @@ internal sealed class JxlExtraChannelInfo : IJxlFields
 
     public int CfaChannel { get; set; }
 
-    public bool Visit(JxlVisitor visitor) => throw new NotImplementedException();
+    public bool Visit(JxlVisitor visitor)
+    {
+        if (visitor.AllDefault(this, ref this.allDefault))
+        {
+            visitor.SetDefault(this);
+            return true;
+        }
+
+        if (!visitor.Enum(JxlExtraChannel.Alpha, ref this.Type))
+        {
+            return false;
+        }
+    }
 }