Fix Decoding interlaced grayscale png's

[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

Basically similar to #830. I missed one spot with the same issue. I have added more test images to cover those cases.

[codecov]

Codecov Report

Merging #831 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
+ Coverage    88.8%   88.84%   +0.03%     
==========================================
  Files        1015     1015              
  Lines       44148    44148              
  Branches     3180     3180              
==========================================
+ Hits        39206    39222      +16     
+ Misses       4212     4201      -11     
+ Partials      730      725       -5

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs	100% <ø> (ø)	⬆️
src/ImageSharp/Formats/Png/PngScanlineProcessor.cs	72.82% <100%> (+4.18%)	⬆️
...eSharp.Tests/Formats/Png/PngDecoderTests.Chunks.cs	100% <0%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngDecoderCore.cs	85.82% <0%> (+1.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 637f767...cbc8d2a. Read the comment docs.

[codecov]

Codecov Report

Merging #831 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #831      +/-   ##
==========================================
+ Coverage    88.8%   88.84%   +0.03%     
==========================================
  Files        1015     1015              
  Lines       44148    44148              
  Branches     3180     3180              
==========================================
+ Hits        39206    39222      +16     
+ Misses       4212     4201      -11     
+ Partials      730      725       -5

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...ts/ImageSharp.Tests/Formats/Png/PngDecoderTests.cs	100% <ø> (ø)	⬆️
src/ImageSharp/Formats/Png/PngScanlineProcessor.cs	72.82% <100%> (+4.18%)	⬆️
...eSharp.Tests/Formats/Png/PngDecoderTests.Chunks.cs	100% <0%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngDecoderCore.cs	85.82% <0%> (+1.01%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 637f767...cbc8d2a. Read the comment docs.

[JimBobSquarePants]

Nice work. I had trouble sourcing test images for this myself.