Implement IEquatable<Block8x8F> and check when summing.

[JimBobSquarePants]

Prerequisites

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• I have provided test coverage for my change (where applicable)

Description

Fix #845

[codecov]

Codecov Report

Merging #848 into master will decrease coverage by 0.08%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #848      +/-   ##
==========================================
- Coverage   88.88%   88.79%   -0.09%     
==========================================
  Files        1014     1014              
  Lines       44272    44289      +17     
  Branches     3206     3207       +1     
==========================================
- Hits        39349    39325      -24     
- Misses       4201     4203       +2     
- Partials      722      761      +39

Impacted Files	Coverage Δ
tests/ImageSharp.Tests/TestImages.cs	100% <ø> (ø)	⬆️
...arp.Tests/Formats/Jpg/JpegDecoderTests.MetaData.cs	100% <ø> (ø)	⬆️
...rc/ImageSharp/Formats/Jpeg/Components/Block8x8F.cs	87.13% <100%> (+0.91%)	⬆️
...ormats/Jpeg/Components/Decoder/QualityEvaluator.cs	94.04% <100%> (+2.48%)	⬆️
...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%)	⬇️
... and 18 more

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[antonfirsov]

LGTM! 👍