Fix issue in ExtractAlphaRows: Filter needs to be applied after extract green

src/ImageSharp/Formats/WebP/AlphaDecoder.cs

@@ -261,8 +261,8 @@ namespace SixLabors.ImageSharp.Formats.Experimental.WebP
             // Extract alpha (which is stored in the green plane).
             int pixelCount = width * numRowsToProcess;
             WebPLosslessDecoder.ApplyInverseTransforms(dec, input, this.memoryAllocator);
-            this.AlphaApplyFilter(0, numRowsToProcess, output, width);
             ExtractGreen(input, output, pixelCount);
+            this.AlphaApplyFilter(0, numRowsToProcess, output, width);
         }
 
         private static void ColorIndexInverseTransformAlpha(

tests/ImageSharp.Tests/Formats/WebP/WebPDecoderTests.cs

@@ -189,6 +189,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.WebP
         [WithFile(Lossy.Alpha2, PixelTypes.Rgba32)]
         [WithFile(Lossy.Alpha3, PixelTypes.Rgba32)]
         [WithFile(Lossy.AlphaThinkingSmiley, PixelTypes.Rgba32)]
+        [WithFile(Lossy.AlphaSticker, PixelTypes.Rgba32)]
         public void WebpDecoder_CanDecode_Lossy_WithAlpha<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel>
         {

tests/ImageSharp.Tests/TestImages.cs

@@ -641,6 +641,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string AlphaNoCompressionGradientFilter = "WebP/alpha_filter_3_method_0.webp";
                 public const string AlphaCompressedGradientFilter = "WebP/alpha_filter_3_method_1.webp";
                 public const string AlphaThinkingSmiley = "WebP/1602311202.webp";
+                public const string AlphaSticker = "WebP/Sticker.webp";
             }
         }
     }

tests/Images/Input/WebP/sticker.webp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:49795fc80522dae2ca687b345c21e9b0848f307d3cc3e39fbdcda730772d338c
+size 27734