Add JPEG parsed data

src/ImageSharp/Formats/Jxl/Processing/Jpeg/JpegAppMarkerType.cs → src/ImageSharp/Formats/Jxl/Processing/Jpeg/Data/JpegAppMarkerType.cs

@@ -1,7 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Six Labors Split License.
 
-namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Jpeg;
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Jpeg.Data;
 
 /// <summary>
 /// Identifies the kind of APP marker in a JPEG file.

src/ImageSharp/Formats/Jxl/Processing/Jpeg/Data/JpegDataConstants.cs

@@ -0,0 +1,20 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+namespace SixLabors.ImageSharp.Formats.Jxl.Processing.Jpeg.Data;
+
+/// <summary>
+/// Constants used in parsed JPEG data.
+/// </summary>
+internal static class JpegDataConstants
+{
+    /// <summary>
+    /// Maximum number of components.
+    /// </summary>
+    public const int MaxComponents = 4;
+
+    /// <summary>
+    /// Maximum number of quantizer tables.
+    /// </summary>
+    public const int MaximumQuantizationTables = 4;
+}

src/ImageSharp/Formats/Jxl/Processing/Jpeg/README.md

@@ -0,0 +1,9 @@
+# Jxl/Processing/Jpeg
+This folder contains logic to:
+
+- Parse and represent JPEG data
+- Write JPEG markers
+- JPEG to JXL
+- JXL to JPEG
+
+This does not contain a JPEG codec.