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

src/ImageSharp/Formats/Png/PngScanlineProcessor.cs

@@ -240,9 +240,9 @@ namespace SixLabors.ImageSharp.Formats.Png
             else
             {
                 Rgba32 rgba32 = default;
+                int offset = 0;
                 for (int x = pixelOffset; x < header.Width; x += increment)
                 {
-                    int offset = x * bytesPerPixel;
                     byte luminance = Unsafe.Add(ref scanlineSpanRef, offset);
                     byte alpha = Unsafe.Add(ref scanlineSpanRef, offset + bytesPerSample);
                     rgba32.R = luminance;
@@ -252,6 +252,7 @@ namespace SixLabors.ImageSharp.Formats.Png
 
                     pixel.FromRgba32(rgba32);
                     Unsafe.Add(ref rowSpanRef, x) = pixel;
+                    offset += bytesPerPixel;
                 }
             }
         }

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

@@ -67,6 +67,12 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             TestImages.Png.GrayTrns16BitInterlaced
         };
 
+        public static readonly string[] TestImagesGrayAlpha8Bit =
+            {
+                TestImages.Png.GrayAlpha8Bit,
+                TestImages.Png.GrayAlpha8Bit2
+            };
+
         public static readonly TheoryData<string, int, int, PixelResolutionUnit> RatioFiles =
         new TheoryData<string, int, int, PixelResolutionUnit>
         {
@@ -123,6 +129,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             }
         }
 
+        [Theory]
+        [WithFileCollection(nameof(TestImagesGrayAlpha8Bit), PixelTypes.Rgba32)]
+        public void Decoder_Gray8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, ImageComparer.Exact);
+            }
+        }
+
         [Theory]
         [WithFileCollection(nameof(TestImagesGray16Bit), PixelTypes.Rgb48)]
         public void Decode_Gray16Bit<TPixel>(TestImageProvider<TPixel> provider)
@@ -159,6 +177,18 @@ namespace SixLabors.ImageSharp.Tests.Formats.Png
             }
         }
 
+        [Theory]
+        [WithFile(TestImages.Png.GrayAlpha8Bit2, PixelTypes)]
+        public void Decoder_CanDecodeGrey8bitWithAlpha<TPixel>(TestImageProvider<TPixel> provider)
+            where TPixel : struct, IPixel<TPixel>
+        {
+            using (Image<TPixel> image = provider.GetImage(new PngDecoder()))
+            {
+                image.DebugSave(provider);
+                image.CompareToOriginal(provider, ImageComparer.Exact);
+            }
+        }
+
         [Fact]
         public void Decode_IgnoreMetadataIsFalse_TextChunckIsRead()
         {

tests/ImageSharp.Tests/TestImages.cs

@@ -29,6 +29,7 @@ namespace SixLabors.ImageSharp.Tests
             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";