Add test

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

@@ -87,7 +87,8 @@ public partial class PngDecoderTests
         TestImages.Png.DisposeBackgroundRegion,
         TestImages.Png.DisposePreviousFirst,
         TestImages.Png.DisposeBackgroundBeforeRegion,
-        TestImages.Png.BlendOverMultiple
+        TestImages.Png.BlendOverMultiple,
+        TestImages.Png.FrameOffset
     };
 
     [Theory]

tests/ImageSharp.Tests/TestImages.cs

@@ -73,6 +73,7 @@ public static class TestImages
         public const string DisposeBackgroundRegion = "Png/animated/15-dispose-background-region.png";
         public const string DisposePreviousFirst = "Png/animated/12-dispose-prev-first.png";
         public const string BlendOverMultiple = "Png/animated/21-blend-over-multiple.png";
+        public const string FrameOffset = "Png/animated/frame-offset.png";
         public const string Issue2666 = "Png/issues/Issue_2666.png";
 
         // Filtered test images from http://www.schaik.com/pngsuite/pngsuite_fil_png.html

tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/00.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:84e2353264e3488122f4d488d7c4b198ff5192ad0c662c7fb0a369c957ecc7ea
+size 353

tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/01.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:2916711d3f4d72eb66a5cfc2b40a3318eb4cce5b367658cfc7e3b573fd39cc33
+size 693

tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/02.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:7f9d5503414ccefa6b66661b1e93c2c3f6e4491f14af006a71153cecf43b52f5
+size 806

tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/03.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:fe42b7dc6524d5589ad680650f4bcd181319b40b258b31e0932d6e936818e980
+size 570

tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_frame-offset.png/04.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:8002ff5b3451b348f285eec15dd7a093c62d11d8b77c3ead9ac89ca6eb29977d
+size 669

tests/Images/Input/Png/animated/frame-offset.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:3c019073841b48b02cb07c779fed8654c6052aee700e7620d07f5d775d97088f
+size 2156