Fix Decoding 8-Bit interlaced grayscale png's with alpha channel

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

This PR fixes an issue with decoding 8-Bit interlaced grayscale images. The offset was not calculated correctly.

Here is an example image:

This was decoded as:

[codecov]

Codecov Report

Merging #830 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #830      +/-   ##
==========================================
- Coverage   88.74%   88.71%   -0.04%     
==========================================
  Files        1015     1015              
  Lines       44121    44138      +17     
  Branches     3178     3178              
==========================================
+ Hits        39157    39158       +1     
+ Misses       4236     4216      -20     
- Partials      728      764      +36

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs	100% <100%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngScanlineProcessor.cs	68.64% <100%> (+5.35%)	⬆️
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.34% <0%> (-9.84%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	88.29% <0%> (-5.32%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	82.05% <0%> (-5.13%)	⬇️
.../Processing/Processors/Transforms/CropProcessor.cs	96% <0%> (-4%)	⬇️
...ocessing/Processors/Dithering/ErrorDiffuserBase.cs	93.75% <0%> (-3.13%)	⬇️
...rs/Normalization/HistogramEqualizationProcessor.cs	97.5% <0%> (-2.5%)	⬇️
...eSharp.Tests/IO/DoubleBufferedStreamReaderTests.cs	97.61% <0%> (-2.39%)	⬇️
... and 18 more

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[JimBobSquarePants]

@brianpopow Perfect! Thanks very much for this 👍

[brianpopow]

@JimBobSquarePants: you are welcome