Add AVX2 version of Vp8_Sse4X4

[brianpopow]

Prerequisites

• I have written a descriptive pull-request title
• I have verified that there are no overlapping pull-requests open
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• I have provided test coverage for my change (where applicable)

Description

This PR adds a AVX2 version of Vp8_Sse4X4, which is used during lossy webp encoding.

Related to #1786

Profiler results:

master:

PR

Test image was Calliphora.png

[codecov]

Codecov Report

Merging #1860 (4a9da2f) into master (16ea95b) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #1860   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         937     937           
  Lines       48847   48868   +21     
  Branches     6096    6097    +1     
======================================
+ Hits        42622   42647   +25     
+ Misses       5213    5210    -3     
+ Partials     1012    1011    -1     

Flag	Coverage Δ
unittests	87% <100%> (+<1%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...rc/ImageSharp/Formats/Webp/Lossy/Vp8EncIterator.cs	100% <ø> (ø)
src/ImageSharp/Formats/Webp/Lossy/LossyUtils.cs	100% <100%> (ø)
...mageSharp/Formats/Webp/Lossless/NearLosslessEnc.cs	96% <0%> (+7%)	⬆️

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[JimBobSquarePants]

LGTM 👍