Fix Decoding 8-Bit grayscale png's with alpha (SixLabors#830)

Author: brianpopow

tests/ImageSharp.Tests/TestImages.cs

@@ -29,6 +29,7 @@ public static class Png
             public const string Gray4Bpp = "Png/gray_4bpp.png";
             public const string Gray16Bit = "Png/gray-16.png";
             public const string GrayAlpha8Bit = "Png/gray-alpha-8.png";
+            public const string GrayAlpha8Bit2 = "Png/rollsroyce.png";
             public const string GrayAlpha16Bit = "Png/gray-alpha-16.png";
             public const string GrayTrns16BitInterlaced = "Png/gray-16-tRNS-interlaced.png";
             public const string Rgb24BppTrans = "Png/rgb-8-tRNS.png";