Clipper upstream fixes

[JimBobSquarePants]

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

Updates the polygon clipping code to include all relevant upstream fixes.
Also includes some minor cleanup.

[codecov]

Codecov Report

Merging #282 (85ec501) into main (072ecd7) will increase coverage by 1%.
The diff coverage is 42%.

@@         Coverage Diff         @@
##           main   #282   +/-   ##
===================================
+ Coverage    79%    80%   +1%     
===================================
  Files        97     97           
  Lines      5088   5020   -68     
  Branches    947    919   -28     
===================================
+ Hits       4021   4025    +4     
+ Misses      868    796   -72     
  Partials    199    199           

Flag	Coverage Δ
unittests	80% <42%> (+1%)	⬆️

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Files Changed	Coverage Δ
...ImageSharp.Drawing/Processing/PathGradientBrush.cs	82% <ø> (-1%)	⬇️
...mageSharp.Drawing/Shapes/PolygonClipper/BoundsF.cs	0% <0%> (ø)
...harp.Drawing/Shapes/PolygonClipper/ClipperUtils.cs	39% <37%> (-3%)	⬇️
...rp.Drawing/Shapes/PolygonClipper/PolygonClipper.cs	68% <75%> (+3%)	⬆️

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