Add tests for PatterBrush issue 241

[JimBobSquarePants]

Prerequisites

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

Description

Fixes #241
This must have been fixed in the rewrite of the brushes. I've added the test from the issue and removed some unused fields from the brush.

[codecov]

Codecov Report

Merging #274 (746f10f) into main (83747f4) will decrease coverage by 1%.
The diff coverage is n/a.

@@         Coverage Diff         @@
##           main   #274   +/-   ##
===================================
- Coverage    78%    78%   -1%     
===================================
  Files        97     97           
  Lines      5067   5065    -2     
  Branches    932    932           
===================================
- Hits       3985   3983    -2     
  Misses      898    898           
  Partials    184    184           

Flag	Coverage Δ
unittests	78% <ø> (-1%)	⬇️

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Impacted Files	Coverage Δ
src/ImageSharp.Drawing/Processing/PatternBrush.cs	76% <ø> (-1%)	⬇️

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