Remove Enum Comparison Boxing

[JimBobSquarePants]

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Description

Replaces enum.Equals(object) with enum == enum. Fixes #862

[codecov]

Codecov Report

Merging #863 into master will decrease coverage by 0.1%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master     #863      +/-   ##
==========================================
- Coverage   88.89%   88.79%   -0.11%     
==========================================
  Files        1014     1014              
  Lines       44295    44295              
  Branches     3209     3209              
==========================================
- Hits        39376    39331      -45     
- Misses       4198     4203       +5     
- Partials      721      761      +40

Impacted Files	Coverage Δ
src/ImageSharp/Formats/Gif/GifEncoderCore.cs	93.2% <100%> (ø)	⬆️
src/ImageSharp/Formats/Png/PngEncoderCore.cs	93.24% <60%> (-0.28%)	⬇️
...xelFormats/PixelOperations{TPixel}.PixelBenders.cs	89.34% <0%> (-9.84%)	⬇️
...Processing/Processors/Transforms/TransformUtils.cs	88.29% <0%> (-5.32%)	⬇️
src/ImageSharp/Common/Helpers/ImageMaths.cs	82.05% <0%> (-5.13%)	⬇️
.../Processing/Processors/Transforms/CropProcessor.cs	96% <0%> (-4%)	⬇️
...ocessing/Processors/Dithering/ErrorDiffuserBase.cs	93.75% <0%> (-3.13%)	⬇️
...rs/Normalization/HistogramEqualizationProcessor.cs	97.5% <0%> (-2.5%)	⬇️
...ormats/Jpeg/Components/Decoder/QualityEvaluator.cs	94.04% <0%> (-2.39%)	⬇️
...eSharp.Tests/IO/DoubleBufferedStreamReaderTests.cs	97.61% <0%> (-2.39%)	⬇️
... and 16 more

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