Stub color table mode enum.

src/ImageSharp/Formats/Gif/GifColorTableMode.cs

@@ -0,0 +1,21 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Formats.Gif
+{
+    /// <summary>
+    /// Provides enumeration for the available Gif color table modes.
+    /// </summary>
+    public enum GifColorTableMode
+    {
+        /// <summary>
+        /// A single color table is calculated from the first frame and reused for subsequent frames.
+        /// </summary>
+        Global,
+
+        /// <summary>
+        /// A unique color table is calculated for each frame.
+        /// </summary>
+        Local
+    }
+}