Ignore invalid gamma chunks

src/ImageSharp/Formats/Png/PngDecoderCore.cs

@@ -432,7 +432,8 @@ namespace SixLabors.ImageSharp.Formats.Png
         {
             if (data.Length < 4)
             {
-                PngThrowHelper.ThrowInvalidGamma();
+                // Ignore invalid gamma chunks.
+                return;
             }
 
             // For example, a gamma of 1/2.2 would be stored as 45455.

src/ImageSharp/Formats/Png/PngThrowHelper.cs

@@ -24,9 +24,6 @@ namespace SixLabors.ImageSharp.Formats.Png
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowMissingPalette() => throw new InvalidImageContentException("PNG Image does not contain a palette chunk");
 
-        [MethodImpl(InliningOptions.ColdPath)]
-        public static void ThrowInvalidGamma() => throw new InvalidImageContentException("PNG Image does not contain enough data for the gamma chunk");
-
         [MethodImpl(InliningOptions.ColdPath)]
         public static void ThrowInvalidChunkType() => throw new InvalidImageContentException("Invalid PNG data.");
 

tests/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs

@@ -283,7 +283,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
 
         [Theory]
         [WithFile(TestImages.Png.Bad.InvalidGammaChunk, PixelTypes.Rgba32)]
-        public void Decode_InvalidGammaChunk_ThrowsException<TPixel>(TestImageProvider<TPixel> provider)
+        public void Decode_InvalidGammaChunk_Ignored<TPixel>(TestImageProvider<TPixel> provider)
             where TPixel : unmanaged, IPixel<TPixel>
         {
             Exception ex = Record.Exception(
@@ -292,8 +292,7 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
                     using Image<TPixel> image = provider.GetImage(PngDecoder);
                     image.DebugSave(provider);
                 });
-            Assert.NotNull(ex);
-            Assert.Contains("PNG Image does not contain enough data for the gamma chunk", ex.Message);
+            Assert.Null(ex);
         }
 
         [Theory]