Prevent a subpixel being able to reset dirty flag

[tocsoft]

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Description

When rasterizing scanlines where the intersection with the region (any where on the line) is less than a pixel wide we had a bug that would reset the scanlineDirty. it should only ever be possible to flip the bit to true and never false in the region of changed code.

How this ended up manifesting is when drawing a shape (seen usually in text) of a pixel wide you would see antialias artefacts even with it disabled.

Before

After

resolves #203

[codecov]

Codecov Report

Merging #204 (d090e0a) into main (6b6c60f) will not change coverage.
The diff coverage is 100%.

@@         Coverage Diff         @@
##           main   #204   +/-   ##
===================================
  Coverage    70%    70%           
===================================
  Files        87     87           
  Lines      5117   5117           
  Branches   1062   1062           
===================================
  Hits       3628   3628           
  Misses     1279   1279           
  Partials    210    210           

Flag	Coverage Δ
unittests	70% <100%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...awing/Shapes/Rasterization/RasterizerExtensions.cs	100% <100%> (ø)

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[JimBobSquarePants]

@tocsoft Looks like a few ref images might actually need updating. I've got the solution open now so will test and commit.

[JimBobSquarePants]

LGTM 👍