Fix Out of Bounds Text Drawing

[JimBobSquarePants]

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Description

Fixes #301

The DrawTextProcessor allowed rendered buffers that exceeded the width of the image frame. This PR clamps the length of the span passed to the brush applicator. Our other processors correctly limit the span length.

I also added colorstop stable sorting to the gradient brush since I spotted the TODO while debugging.

[codecov]

Codecov Report

Merging #304 (758b7b4) into main (85721f8) will decrease coverage by 1%.
The diff coverage is 85%.

@@         Coverage Diff         @@
##           main   #304   +/-   ##
===================================
- Coverage    80%    80%   -1%     
===================================
  Files        97     97           
  Lines      4937   4946    +9     
  Branches    881    884    +3     
===================================
+ Hits       3990   3997    +7     
- Misses      753    754    +1     
- Partials    194    195    +1     

Flag	Coverage Δ
unittests	80% <85%> (-1%)	⬇️

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Files	Coverage Δ
...ssing/Processors/Text/DrawTextProcessor{TPixel}.cs	80% <100%> (ø)
src/ImageSharp.Drawing/Processing/GradientBrush.cs	81% <84%> (-1%)	⬇️

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