Adding missed interface

Former-commit-id: 3dcdd087c3e70b7aef92425bc9c6ba0fe96dd292
Former-commit-id: fd5dd1856cf371005622acebc9e49b3d866f87a0

src/ImageProcessor/ImageProcessor.csproj

@@ -159,6 +159,7 @@
     <Compile Include="Common\Exceptions\ImageFormatException.cs" />
     <Compile Include="ImageFactory.cs" />
     <Compile Include="Imaging\AnchorPosition.cs" />
+    <Compile Include="Imaging\Formats\IIndexedImageFormat.cs" />
     <Compile Include="Imaging\Helpers\ImageMaths.cs" />
     <Compile Include="Imaging\ImageLayer.cs" />
     <Compile Include="Imaging\MetaData\ExifPropertyTagType.cs" />

src/ImageProcessor/Imaging/Formats/IQuantizableImageFormat.cs

@@ -0,0 +1,25 @@
+// --------------------------------------------------------------------------------------------------------------------
+// <copyright file="IIndexedImageFormat.cs" company="James South">
+//   Copyright (c) James South.
+//   Licensed under the Apache License, Version 2.0.
+// </copyright>
+// <summary>
+//   The IQuantizableImageFormat interface for identifying quantizable image formats.
+// </summary>
+// --------------------------------------------------------------------------------------------------------------------
+
+namespace ImageProcessor.Imaging.Formats
+{
+    using ImageProcessor.Imaging.Quantizers;
+
+    /// <summary>
+    /// The IndexedImageFormat interface for identifying quantizable image formats.
+    /// </summary>
+    public interface IQuantizableImageFormat
+    {
+        /// <summary>
+        /// Gets or sets the quantizer for reducing the image palette.
+        /// </summary>
+        IQuantizer Quantizer { get; set; }
+    }
+}