Add SolidBrush<TPixel> to paint a pixel value

[calrsom]

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Description

Implemented a variation of SolidBrush that takes color as TPixel rather than Color. This allows for painting that uses full precision of RgbaVector and other floating-point pixel formats.

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[codecov[bot]]

Codecov Report

Merging #166 (30b6e95) into master (7ffae70) will decrease coverage by 0.03%. The diff coverage is 83.78%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   70.03%   69.99%   -0.04%     
==========================================
  Files          89       89              
  Lines        5209     5216       +7     
  Branches     1062     1063       +1     
==========================================
+ Hits         3648     3651       +3     
- Misses       1347     1350       +3     
- Partials      214      215       +1     

Flag	Coverage Δ
unittests	69.99% <83.78%> (-0.04%)	:arrow_down:

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Impacted Files	Coverage Δ
src/ImageSharp.Drawing/Processing/SolidBrush.cs	85.36% <83.78%> (-8.76%)	:arrow_down:

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[JimBobSquarePants]

Thanks for this, I'll get it reviewed asap.

[JimBobSquarePants]

Note: Please don't merge this yet until we've had further discussion

[calrsom]

I believe this is handled better by https://github.com/SixLabors/ImageSharp/pull/1801. Also, I ended up needing to further customize the blending math and created a custom IBrush implementation, specific to my custom pixel type.