Add tests for lossless images without transforms

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

@@ -2,6 +2,12 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+
+using SixLabors.ImageSharp.Formats.Bmp;
+using SixLabors.ImageSharp.Formats.WebP;
+using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Tests.TestUtilities.ReferenceCodecs;
+
 using Xunit;
 
 // ReSharper disable InconsistentNaming
@@ -43,5 +49,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.WebP
                 Assert.Equal(expectedHeight, image.Height);
             }
         }
+
+        [Theory]
+        [WithFile(Lossless.LosslessNoTransform1, PixelTypes.Rgba32)]
+        [WithFile(Lossless.LosslessNoTransform2, PixelTypes.Rgba32)]
+        public void WebpDecoder_CanDecode_Lossless_WithoutTransforms<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new WebPDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, new MagickReferenceDecoder());
+            }
+        }
     }
 }

tests/ImageSharp.Tests/TestImages.cs

@@ -382,6 +382,8 @@ namespace SixLabors.ImageSharp.Tests
                 public const string Lossless2 = "WebP/lossless2.webp";
                 public const string Lossless3 = "WebP/lossless3.webp";
                 public const string Lossless4 = "WebP/lossless4.webp";
+                public const string LosslessNoTransform1 = "WebP/lossless_vec_1_0.webp";
+                public const string LosslessNoTransform2 = "WebP/lossless_vec_2_0.webp";
             }
 
             public static class Lossy