Merge SixLabors.Core sources into ImageSharp

[antonfirsov]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
• I have verified that I am following matches the existing coding patterns and practice as demonstrated in the repository. These follow strict Stylecop rules 👮.
• I have provided test coverage for my change (where applicable)

Description

Redo #1086: Merge sources and tests from SixLabors.Core, resolve #1002.

• No namespace adjustments for now, a separate PR should follow instead
• CI improvements from @JimBobSquarePants were cherry picked from Merge SixLabors.Core sources into ImageSharp [first attempt] #1086
• Merging a rewritten history SixLabors.Core for a better git log in ImageSharp repository:
  • All commits to core are prefixed with [SL.Core]
  • Prefixed issue references with repository name to avoid confusion
  • Done with a simple LibGit2Sharp app

[antonfirsov]

Helper tests were left out this time, opened SixLabors/SharedInfrastructure#5 instead.

[codecov]

Codecov Report

Merging #1087 into master will increase coverage by 0.05%.
The diff coverage is 87.33%.

@@            Coverage Diff             @@
##           master    #1087      +/-   ##
==========================================
+ Coverage   83.71%   83.77%   +0.05%     
==========================================
  Files         688      704      +16     
  Lines       28853    29311     +458     
==========================================
+ Hits        24155    24555     +400     
- Misses       4698     4756      +58

Flag	Coverage Δ
#unittests	83.77% <87.33%> (+0.05%)	⬆️

Impacted Files	Coverage Δ
src/ImageSharp/Memory/BufferArea{T}.cs	97.05% <ø> (ø)	⬆️
...ing/Processors/Transforms/CropProcessor{TPixel}.cs	100% <ø> (ø)	⬆️
...s/Convolution/Convolution2PassProcessor{TPixel}.cs	84% <ø> (ø)	⬆️
...ng/Processors/Overlays/BackgroundColorProcessor.cs	100% <ø> (ø)	⬆️
...sors/Binarization/BinaryErrorDiffusionProcessor.cs	88.23% <ø> (ø)	⬆️
...cessing/Processors/Convolution/PrewittProcessor.cs	100% <ø> (ø)	⬆️
...mageSharp/Processing/ProjectiveTransformBuilder.cs	81.39% <ø> (ø)	⬆️
...g/Processors/Filters/LomographProcessor{TPixel}.cs	100% <ø> (ø)	⬆️
...cessing/Processors/Convolution/BoxBlurProcessor.cs	100% <ø> (ø)	⬆️
...ssors/Binarization/BinaryOrderedDitherProcessor.cs	100% <ø> (ø)	⬆️
... and 260 more

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[JimBobSquarePants]

Looks great, well done!

I added the namespace changes in this PR as I need to unblock the version updates to the dependent libraries.