Add convenience constructor to ComplexPolygon

[stefannikolei]

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

Convenience constructors for shapes #100

[codecov[bot]]

Codecov Report

Merging #258 (76a3d8d) into main (6a24a19) will decrease coverage by 1%. The diff coverage is 0%.

@@         Coverage Diff         @@
##           main   #258   +/-   ##
===================================
- Coverage    71%    71%   -1%     
===================================
  Files        88     88           
  Lines      5370   5372    +2     
  Branches   1098   1098           
===================================
  Hits       3853   3853           
- Misses     1298   1300    +2     
  Partials    219    219           

Flag	Coverage Δ
unittests	71% <0%> (-1%)	:arrow_down:

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Impacted Files	Coverage Δ
src/ImageSharp.Drawing/Shapes/ComplexPolygon.cs	78% <0%> (-3%)	:arrow_down:

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