Fix ImageBrush and DrawTextProcessor

[JimBobSquarePants]

Prerequisites

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Description

Fixes #270

[codecov]

Codecov Report

Merging #273 (1b25598) into main (626900c) will not change coverage.
The diff coverage is 0%.

@@         Coverage Diff         @@
##           main   #273   +/-   ##
===================================
  Coverage    78%    78%           
===================================
  Files        97     97           
  Lines      5067   5067           
  Branches    932    932           
===================================
  Hits       3985   3985           
  Misses      898    898           
  Partials    184    184           

Flag	Coverage Δ
unittests	78% <0%> (ø)

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Impacted Files	Coverage Δ
src/ImageSharp.Drawing/Processing/ImageBrush.cs	82% <0%> (ø)
...ssing/Processors/Text/DrawTextProcessor{TPixel}.cs	80% <0%> (ø)

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