Use per-edge buffer + minor optimizations.

[JimBobSquarePants]

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Description

Added pooled buffer generation to the edge class to make it threadsafe. Also did some very low hanging optimization fruit. Fix #1044

@jongleur1983 can you also have a look at the fix please to confirm. Thanks!

[codecov]

Codecov Report

Merging #1079 into master will decrease coverage by <.01%.
The diff coverage is 77.27%.

@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
- Coverage   89.96%   89.95%   -0.01%     
==========================================
  Files        1200     1200              
  Lines       53010    53035      +25     
  Branches     3844     3846       +2     
==========================================
+ Hits        47689    47708      +19     
- Misses       4554     4560       +6     
  Partials      767      767

Impacted Files	Coverage Δ
...harp.Tests/Drawing/FillLinearGradientBrushTests.cs	99.62% <100%> (+0.01%)	⬆️
...essing/Processors/Drawing/FillProcessor{TPixel}.cs	93.54% <100%> (ø)	⬆️
...c/ImageSharp.Drawing/Processing/BrushApplicator.cs	84.37% <37.5%> (-5.29%)	⬇️
...ImageSharp.Drawing/Processing/PathGradientBrush.cs	88% <79.16%> (-3.02%)	⬇️

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[jongleur1983]

Didn't test myself, but the code changes look good.
As I suggest the old tests still pass and the new one looks reasonable to reproduce the issue I approve these changes without testing myself.