Fix arcs and do not throw when outlining

[JimBobSquarePants]

Prerequisites

• [x] I have written a descriptive pull-request title
• [x] I have verified that there are no overlapping pull-requests open
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• [x] I have provided test coverage for my change (where applicable)

Description

Fixes #299, #300

The way I was handling arc generation when the arc sweep represented a closed ellipse didn't take into consideration the start angle.

Fixing the issue meant that we threw during outlining when someone requested the short arc so I fixed the offsetter to return the unclipped path I also fixed the handling of paths containing a single point.

[codecov[bot]]

Codecov Report

Merging #302 (edb6db2) into main (dfae51c) will increase coverage by 0%. The diff coverage is 92%.

@@         Coverage Diff         @@
##           main   #302   +/-   ##
===================================
  Coverage    80%    80%           
===================================
  Files        97     97           
  Lines      4937   4937           
  Branches    878    881    +3     
===================================
+ Hits       3975   3990   +15     
+ Misses      769    753   -16     
- Partials    193    194    +1     

Flag	Coverage Δ
unittests	80% <92%> (+<1%)	:arrow_up:

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Files	Coverage Δ
src/ImageSharp.Drawing/Shapes/ArcLineSegment.cs	96% <100%> (+<1%)	:arrow_up:
...mageSharp.Drawing/Shapes/PolygonClipper/BoundsF.cs	28% <ø> (+28%)	:arrow_up:
....Drawing/Shapes/PolygonClipper/PolygonOffsetter.cs	74% <60%> (+1%)	:arrow_up:

... and 2 files with indirect coverage changes

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