Add test making sure compressed alpha data is smaller then image size

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

@@ -283,6 +283,26 @@ namespace SixLabors.ImageSharp.Tests.Formats.Webp
             image.VerifyEncoder(provider, "webp", $"with_alpha_compressed_{compressed}", encoder, ImageComparer.Tolerant(0.04f));
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.Transparency, PixelTypes.Rgba32)]
+        public void Encode_Lossy_WithCompressedAlpha_AlphaShouldBeSmaller<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            var encoder = new WebpEncoder()
+            {
+                FileFormat = WebpFileFormatType.Lossy,
+                UseAlphaCompression = true
+            };
+
+            using Image<TPixel> image = provider.GetImage();
+            using (var memoryStream = new MemoryStream())
+            {
+                image.SaveAsWebp(memoryStream, encoder);
+                int size = memoryStream.ToArray().Length;
+                Assert.True(size < 16300);
+            }
+        }
+
         [Theory]
         [WithFile(TestPatternOpaque, PixelTypes.Rgba32)]
         [WithFile(TestPatternOpaqueSmall, PixelTypes.Rgba32)]