add failing test for global quantization

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

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0.
 
 using System.IO;
+using SixLabors.ImageSharp.Formats;
 using SixLabors.ImageSharp.Formats.Gif;
 using SixLabors.ImageSharp.Metadata;
 using SixLabors.ImageSharp.PixelFormats;
@@ -132,6 +133,30 @@ namespace SixLabors.ImageSharp.Tests.Formats.Gif
             }
         }
 
+        [Theory]
+        [WithFile(TestImages.Gif.GlobalQuantizationTest, PixelTypes.Rgba32)]
+        public void Encode_GlobalPalette_QuantizeMultipleFrames<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : unmanaged, IPixel<TPixel>
+        {
+            using Image<TPixel> image = provider.GetImage();
+
+            var encoder = new GifEncoder()
+            {
+                ColorTableMode = GifColorTableMode.Global
+            };
+
+            string testOutputFile = provider.Utility.SaveTestOutputFile(
+                image,
+                "gif",
+                encoder,
+                appendPixelTypeToFileName: false,
+                appendSourceFileOrDescription: false);
+
+            IImageDecoder referenceDecoder = TestEnvironment.GetReferenceDecoder(testOutputFile);
+            using var encoded = Image.Load<TPixel>(testOutputFile, referenceDecoder);
+            ValidatorComparer.VerifySimilarity(image, encoded);
+        }
+
         [Fact]
         public void NonMutatingEncodePreservesPaletteCount()
         {

tests/ImageSharp.Tests/TestImages.cs

@@ -396,6 +396,7 @@ namespace SixLabors.ImageSharp.Tests
             public const string Ratio4x1 = "Gif/base_4x1.gif";
             public const string Ratio1x4 = "Gif/base_1x4.gif";
             public const string LargeComment = "Gif/large_comment.gif";
+            public const string GlobalQuantizationTest = "Gif/GlobalQuantizationTest.gif";
 
             // Test images from https://github.com/robert-ancell/pygif/tree/master/test-suite
             public const string ZeroSize = "Gif/image-zero-size.gif";

tests/Images/Input/Gif/GlobalQuantizationTest.gif

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:c67df4b08561c14054ed911e6cfc99f1fc726b32e2c7a5e2dbb8392e24109b5a
+size 101868