Upgrade to clipper 2

[JimBobSquarePants]

Prerequisites

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

Description

Replaces Clipper 1 with Clipper 2 which benchmarks around 2X faster. Everythign was copied as-is with the only changes to make all the clipper code internal.

[codecov[bot]]

Codecov Report

Merging #261 (69621fb) into main (2b2f610) will decrease coverage by 11%. The diff coverage is 18%.

@@         Coverage Diff          @@
##           main   #261    +/-   ##
====================================
- Coverage    71%    61%   -11%     
====================================
  Files        95    100     +5     
  Lines      5643   6449   +806     
  Branches   1157   1378   +221     
====================================
- Hits       4020   3934    -86     
- Misses     1387   2314   +927     
+ Partials    236    201    -35     

Flag	Coverage Δ
unittests	61% <18%> (-11%)	:arrow_down:

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

Impacted Files	Coverage Δ
...mageSharp.Drawing/Shapes/Clipper/Clipper.Engine.cs	54% <ø> (ø)
...eSharp.Drawing/Shapes/Clipper/Clipper.Minkowski.cs	0% <0%> (ø)
...geSharp.Drawing/Shapes/Clipper/Clipper.RectClip.cs	0% <0%> (ø)
src/ImageSharp.Drawing/Shapes/Clipper/Clipper.cs	3% <3%> (ø)
.../ImageSharp.Drawing/Shapes/Clipper/Clipper.Core.cs	20% <20%> (ø)
...mageSharp.Drawing/Shapes/PolygonClipper/Clipper.cs	73% <70%> (-6%)	:arrow_down:
...mageSharp.Drawing/Shapes/Clipper/Clipper.Offset.cs	73% <73%> (ø)
...arp.Drawing/Shapes/PolygonClipper/ClipperOffset.cs	100% <100%> (ø)

... and 1 file with indirect coverage changes

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[JimBobSquarePants]

We should remove Clipper from code coverage