Fix issue with decoding VP8 alpha channel: Use8BDecode will only be set to true, if there is a transform present

src/ImageSharp/Formats/WebP/AlphaDecoder.cs

@@ -61,7 +61,7 @@ namespace SixLabors.ImageSharp.Formats.WebP
                 var bitReader = new Vp8LBitReader(data);
                 this.LosslessDecoder = new WebPLosslessDecoder(bitReader, memoryAllocator, configuration);
                 this.LosslessDecoder.DecodeImageStream(this.Vp8LDec, width, height, true);
-                this.Use8BDecode = Is8BOptimizable(this.Vp8LDec.Metadata);
+                this.Use8BDecode = this.Vp8LDec.Transforms.Count > 0 && Is8BOptimizable(this.Vp8LDec.Metadata);
             }
         }
 

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

@@ -181,6 +181,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.WebP
         [WithFile(Lossy.Alpha1, PixelTypes.Rgba32)]
         [WithFile(Lossy.Alpha2, PixelTypes.Rgba32)]
         [WithFile(Lossy.Alpha3, PixelTypes.Rgba32)]
+        [WithFile(Lossy.AlphaThinkingSmiley, PixelTypes.Rgba32)]
         public void WebpDecoder_CanDecode_Lossy_WithAlpha<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel>
         {

tests/ImageSharp.Tests/TestImages.cs

@@ -640,6 +640,7 @@ namespace SixLabors.ImageSharp.Tests
                 public const string AlphaCompressedVerticalFilter = "WebP/alpha_filter_2_method_1.webp";
                 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";
             }
         }
     }

tests/Images/Input/WebP/1602311202.webp

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:4dac76bec0e5b23a988b0f2221e9b20e63dc207ef48f33e49a4336a874e2a915
+size 18406