introduce WebPFilterType enum

src/ImageSharp/Formats/WebP/WebPFilterType.cs

@@ -0,0 +1,17 @@
+// Copyright (c) Six Labors and contributors.
+// Licensed under the Apache License, Version 2.0.
+
+namespace SixLabors.ImageSharp.Formats.WebP
+{
+    // TODO from dsp.h
+    public enum WebPFilterType
+    {
+        None = 0,
+        Horizontal,
+        Vertical,
+        Gradient,
+        Last = Gradient + 1, // end marker
+        Best, // meta types
+        Fast
+    }
+}