Add test for default image not animated

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

@@ -88,7 +88,8 @@ public partial class PngDecoderTests
         TestImages.Png.DisposePreviousFirst,
         TestImages.Png.DisposeBackgroundBeforeRegion,
         TestImages.Png.BlendOverMultiple,
-        TestImages.Png.FrameOffset
+        TestImages.Png.FrameOffset,
+        TestImages.Png.DefaultNotAnimated
     };
 
     [Theory]

tests/ImageSharp.Tests/TestImages.cs

@@ -74,6 +74,7 @@ public static class TestImages
         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 DefaultNotAnimated = "Png/animated/default-not-animated.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_default-not-animated.png/00.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:8d4716e18655be53630d6d50daebe8c38e0eedb2432c7a73840b55d1473d5944
+size 1050

tests/Images/External/ReferenceOutput/PngDecoderTests/Decode_VerifyAllFrames_Rgba32_default-not-animated.png/01.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:9b5a6d3cf1a777f6b719c2a1cf79bffe2251355d75e6c0f7ce7a973b3d033419
+size 1177

tests/Images/Input/Png/animated/default-not-animated.png

@@ -0,0 +1,3 @@
+version https://git-lfs.github.com/spec/v1
+oid sha256:647d484c8f320b55824b9219270524df3edc434a4793e1627e0ee14af8d6e4f8
+size 1689