Add explicit tests for #2450

Author: JimBobSquarePants

src/ImageSharp/Formats/Gif/GifEncoderCore.cs

@@ -290,6 +290,10 @@ private void EncodeFrame<TPixel>(
         }
 
         this.WriteGraphicalControlExtension(metadata, transparencyIndex, stream);
+
+        // TODO: Consider an optimization that trims down the buffer to the minimum size required.
+        // We would use a process similar to entropy crop where we trim the buffer from the edges
+        // until we hit a non-transparent pixel.
         this.WriteImageDescriptor(frame, useLocal, stream);
 
         if (useLocal)

tests/ImageSharp.Tests/Formats/Gif/GifDecoderTests.cs

@@ -34,6 +34,17 @@ public void Decode_VerifyAllFrames<TPixel>(TestImageProvider<TPixel> provider)
         image.CompareToReferenceOutputMultiFrame(provider, ImageComparer.Exact);
     }
 
+    [Theory]
+    [WithFile(TestImages.Gif.Issues.Issue2450_A, PixelTypes.Rgba32)]
+    [WithFile(TestImages.Gif.Issues.Issue2450_B, PixelTypes.Rgba32)]
+    public void Decode_Issue2450<TPixel>(TestImageProvider<TPixel> provider)
+    where TPixel : unmanaged, IPixel<TPixel>
+    {
+        using Image<TPixel> image = provider.GetImage();
+        image.DebugSaveMultiFrame(provider);
+        image.CompareToReferenceOutputMultiFrame(provider, ImageComparer.Exact);
+    }
+
     [Theory]
     [WithFile(TestImages.Gif.Giphy, PixelTypes.Rgba32)]
     public void GifDecoder_Decode_Resize<TPixel>(TestImageProvider<TPixel> provider)

tests/ImageSharp.Tests/TestImages.cs

@@ -484,7 +484,8 @@ public static class Issues
             public const string Issue2288_B = "Gif/issues/issue_2288_2.gif";
             public const string Issue2288_C = "Gif/issues/issue_2288_3.gif";
             public const string Issue2288_D = "Gif/issues/issue_2288_4.gif";
-            public const string Issue2450 = "Gif/issues/issue_2450.gif";
+            public const string Issue2450_A = "Gif/issues/issue_2450.gif";
+            public const string Issue2450_B = "Gif/issues/issue_2450_2.gif";
             public const string Issue2198 = "Gif/issues/issue_2198.gif";
         }