SSE2 / AVX2 of webp AlphaDecoding

[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 SSE2 / AVX2 versions of VerticalUnfilter and HorizontalUnfilter which are used during webp decoding with alpha channels.

Related to #1786

Profiling results:

master:

PR:

[codecov]

Codecov Report

Merging #1930 (6bd756e) into master (dfe3fb5) will decrease coverage by 0%.
The diff coverage is 89%.

❗ Current head 6bd756e differs from pull request most recent head 4c4f27d. Consider uploading reports for the commit 4c4f27d to get more accurate results

@@           Coverage Diff           @@
##           master   #1930    +/-   ##
=======================================
- Coverage      88%     88%    -1%     
=======================================
  Files         968     966     -2     
  Lines       51565   51383   -182     
  Branches     6428    6410    -18     
=======================================
- Hits        45405   45243   -162     
+ Misses       5090    5078    -12     
+ Partials     1070    1062     -8     

Flag	Coverage Δ
unittests	88% <89%> (-1%)	⬇️

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

Impacted Files	Coverage Δ
.../ImageSharp/Formats/Webp/Lossless/LosslessUtils.cs	98% <ø> (ø)
src/ImageSharp/Formats/Webp/AlphaDecoder.cs	89% <89%> (-1%)	⬇️
src/ImageSharp/Formats/Webp/WebpDecoder.cs	33% <0%> (-21%)	⬇️
src/ImageSharp/Formats/Tiff/TiffDecoder.cs	50% <0%> (-19%)	⬇️
...rc/ImageSharp/Metadata/Profiles/Exif/ExifWriter.cs	89% <0%> (-3%)	⬇️
src/ImageSharp/Formats/ImageFormatManager.cs	92% <0%> (-2%)	⬇️
...geSharp/Formats/Tiff/TiffDecoderMetadataCreator.cs	72% <0%> (-2%)	⬇️
...ImageSharp/Formats/Webp/BitWriter/BitWriterBase.cs	91% <0%> (-1%)	⬇️
src/ImageSharp/Formats/Gif/GifEncoderCore.cs	94% <0%> (-1%)	⬇️
...Formats/Gif/Sections/GifGraphicControlExtension.cs	95% <0%> (-1%)	⬇️
... and 25 more

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[brianpopow]

@gfoidl thanks for your review comments!

[JimBobSquarePants]

Nice speedup there! Great work!