Enable PickBestIntra4

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

@@ -49,11 +49,11 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
         [MethodImpl(InliningOptions.ShortMethod)]
         public static void Copy(Span<byte> src, Span<byte> dst, int w, int h)
         {
+            int offset = 0;
             for (int y = 0; y < h; ++y)
             {
-                src.Slice(0, w).CopyTo(dst);
-                src = src.Slice(WebpConstants.Bps);
-                dst = dst.Slice(WebpConstants.Bps);
+                src.Slice(offset, w).CopyTo(dst.Slice(offset, w));
+                offset += WebpConstants.Bps;
             }
         }
 

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

@@ -168,7 +168,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                         Span<byte> tmp = tmpDst;
                         tmpDst = bestBlock;
                         bestBlock = tmp;
-                        tmpLevels.AsSpan().CopyTo(rdBest.YAcLevels);
+                        tmpLevels.CopyTo(rdBest.YAcLevels.AsSpan(it.I4 * 16, 16));
                     }
                 }
 

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

@@ -893,8 +893,7 @@ namespace SixLabors.ImageSharp.Formats.Webp.Lossy
                 QuantEnc.PickBestIntra16(it, ref rd, this.SegmentInfos, this.Proba);
                 if (this.method >= 2)
                 {
-                    // TODO: there is still a bug in PickBestIntra4, therefore disabled.
-                    // QuantEnc.PickBestIntra4(it, ref rd, this.SegmentInfos, this.Proba, this.maxI4HeaderBits);
+                    QuantEnc.PickBestIntra4(it, ref rd, this.SegmentInfos, this.Proba, this.maxI4HeaderBits);
                 }
 
                 QuantEnc.PickBestUv(it, ref rd, this.SegmentInfos, this.Proba);