Fix VP8 issue with quality 20 and method 2: UseSkipProba was ignored

src/ImageSharp/Formats/WebP/Lossy/Vp8Encoder.cs

@@ -400,9 +400,14 @@ namespace SixLabors.ImageSharp.Formats.WebP.Lossy
             it.InitFilter();
             do
             {
+                bool dontUseSkip = !this.Proba.UseSkipProba;
+
                 var info = new Vp8ModeScore();
                 it.Import(y, u, v, yStride, uvStride, width, height, false);
-                if (!this.Decimate(it, info, this.rdOptLevel))
+
+                // Warning! order is important: first call VP8Decimate() and
+                // *then* decide how to code the skip decision if there's one.
+                if (!this.Decimate(it, info, this.rdOptLevel) || dontUseSkip)
                 {
                     this.CodeResiduals(it, info);
                 }

src/ImageSharp/Formats/WebP/Lossy/Vp8MacroBlockInfo.cs

@@ -1,8 +1,11 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
+using System.Diagnostics;
+
 namespace SixLabors.ImageSharp.Formats.WebP.Lossy
 {
+    [DebuggerDisplay("Type: {MacroBlockType}, Alpha: {Alpha}, UvMode: {UvMode}")]
     internal class Vp8MacroBlockInfo
     {
         public Vp8MacroBlockType MacroBlockType { get; set; }