Add AVX version of TransformColor and TransformColorInverse

[brianpopow]

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Description

This PR add AVX2 versions of TransformColor and TransformColorInverse which are used during webp encoding and decoding.
Relates to #1786

Profiling results:

master:

PR:

[codecov]

Codecov Report

Merging #1830 (306d127) into master (2f048ca) will decrease coverage by 0%.
The diff coverage is 99%.

@@          Coverage Diff           @@
##           master   #1830   +/-   ##
======================================
- Coverage      87%     87%   -1%     
======================================
  Files         937     937           
  Lines       48385   48420   +35     
  Branches     6048    6054    +6     
======================================
- Hits        42285   42217   -68     
- Misses       5097    5196   +99     
- Partials     1003    1007    +4     

Flag	Coverage Δ
unittests	87% <99%> (-1%)	⬇️

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Impacted Files	Coverage Δ
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	89% <99%> (-9%)	⬇️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	87% <0%> (-11%)	⬇️
...mageSharp/Formats/Webp/Lossless/NearLosslessEnc.cs	88% <0%> (-8%)	⬇️
...ImageSharp/Formats/Webp/Lossless/Vp8LBitEntropy.cs	100% <0%> (+1%)	⬆️

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[gfoidl]

Suggested change

	int i;
	nint i;

? As well on the other loop-variables to avoid the movsxd machine-instruction.

[brianpopow]

The slice down below requires an int. Is it still worth using nint here and cast with the slice or should i keep it rather int then? The Slice is outside of the loop and only for the leftover.

[gfoidl]

I'd cast to int in the slice. It's actually a nop in machine code.

[brianpopow]

ok will do that then, thx!

[JimBobSquarePants]

@brianpopow I went ahead and did the nint change.

[brianpopow]

thanks, i was unsure in some places, if I should go for it or not.