Add AVX2 version of QuantizeBlock

[brianpopow]

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 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

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

Related to #1786

Profiling results:

master

PR

Test image was Calliphora.png

[codecov]

Codecov Report

Merging #1857 (909525c) into master (f44f11f) will increase coverage by 0%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #1857    +/-   ##
=======================================
  Coverage      87%     87%            
=======================================
  Files         935     935            
  Lines       48751   48786    +35     
  Branches     6081    6083     +2     
=======================================
+ Hits        42620   42724   +104     
+ Misses       5123    5056    -67     
+ Partials     1008    1006     -2     

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
src/ImageSharp/Formats/Webp/Lossy/QuantEnc.cs	96% <100%> (+<1%)	⬆️
...rc/ImageSharp/Formats/Webp/Lossless/Vp8LEncoder.cs	97% <0%> (+<1%)	⬆️
...ImageSharp/Formats/Webp/Lossless/Vp8LBitEntropy.cs	100% <0%> (+1%)	⬆️
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	98% <0%> (+5%)	⬆️
...ageSharp/Formats/Webp/Lossless/PredictorEncoder.cs	97% <0%> (+6%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f44f11f...909525c. Read the comment docs.

[JimBobSquarePants]

LGTM 👍