Add encoding test pattern tests

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

@@ -4,6 +4,7 @@
 using System.IO;
 using SixLabors.ImageSharp.Formats.Webp;
 using SixLabors.ImageSharp.PixelFormats;
+using SixLabors.ImageSharp.Processing;
 using SixLabors.ImageSharp.Tests.TestUtilities.ImageComparison;
 using Xunit;
 using static SixLabors.ImageSharp.Tests.TestImages.WebP;
@@ -96,6 +97,20 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
             image.VerifyEncoder(provider, "webp", testOutputDetails, encoder, customComparer: GetComparer(quality));
         }
 
+        [Theory]
+        [WithTestPatternImages(187, 221, PixelTypes.Rgba32)]
+        [WithTestPatternImages(100, 118, PixelTypes.Rgba32)]
+        public void Encode_Lossy_WorksWithTestPattern<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage();
+
+            // Encoding lossy images with transparency is not yet supported, therefor the test image will be made opaque.
+            image.Mutate(img => img.MakeOpaque());
+            var encoder = new WebpEncoder() { Lossy = true };
+            image.VerifyEncoder(provider, "webp", string.Empty, encoder);
+        }
+
         [Fact]
         public void Encode_Lossless_OneByOnePixel_Works()
         {