Add encode lossy webp with alpha tests

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

@@ -167,18 +167,6 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
             image.VerifyEncoder(provider, "webp", testOutputDetails, encoder);
         }
 
-        [Theory]
-        [WithFile(TestPatternOpaque, PixelTypes.Rgba32)]
-        [WithFile(TestPatternOpaqueSmall, PixelTypes.Rgba32)]
-        public void Encode_Lossless_WorksWithTestPattern<TPixel>(TestImageProvider<TPixel> provider)
-            where TPixel : unmanaged, IPixel<TPixel>
-        {
-            using Image<TPixel> image = provider.GetImage();
-
-            var encoder = new WebpEncoder() { FileFormat = WebpFileFormatType.Lossless };
-            image.VerifyEncoder(provider, "webp", string.Empty, encoder);
-        }
-
         [Fact]
         public void Encode_Lossless_OneByOnePixel_Works()
         {
@@ -279,6 +267,34 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
             image.VerifyEncoder(provider, "webp", testOutputDetails, encoder, customComparer: GetComparer(quality));
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32, false)]
+        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32, true)]
+        public void Encode_Lossy_WithAlpha_Works<TPixel>(TestImageProvider<TPixel> provider, bool compressed)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            var encoder = new WebpEncoder()
+            {
+                FileFormat = WebpFileFormatType.Lossy,
+                UseAlphaCompression = compressed
+            };
+
+            using Image<TPixel> image = provider.GetImage();
+            image.VerifyEncoder(provider, "webp", $"with_alpha_compressed_{compressed}", encoder, ImageComparer.Tolerant(0.04f));
+        }
+
+        [Theory]
+        [WithFile(TestPatternOpaque, PixelTypes.Rgba32)]
+        [WithFile(TestPatternOpaqueSmall, PixelTypes.Rgba32)]
+        public void Encode_Lossless_WorksWithTestPattern<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage();
+
+            var encoder = new WebpEncoder() { FileFormat = WebpFileFormatType.Lossless };
+            image.VerifyEncoder(provider, "webp", string.Empty, encoder);
+        }
+
         [Theory]
         [WithFile(TestPatternOpaque, PixelTypes.Rgba32)]
         [WithFile(TestPatternOpaqueSmall, PixelTypes.Rgba32)]

tests/ImageSharp.Tests/TestImages.cs

@@ -15,6 +15,7 @@ namespace SixLabors.ImageSharp.Tests
     {
         public static class Png
         {
+            public const string Transparency = "Png/transparency.png";
             public const string P1 = "Png/pl.png";
             public const string Pd = "Png/pd.png";
             public const string Blur = "Png/blur.png";

tests/Images/Input/Png/transparency.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:843bea4db378f52935e2f19f60d289df8ebe20ddde3977c63225f1d58a10bd62
+size 48119