Fix vectorized 4:2:0 subsampling

[tkp1n]

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• I have written a descriptive pull-request title
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• I have provided test coverage for my change (where applicable)

Description

Fixes the bug reported by @jrdiver and @JimBobSquarePants in #1547/#1549 introduced in #1517.

[codecov]

Codecov Report

Merging #1551 (1bb7f48) into master (5f8f082) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1551      +/-   ##
==========================================
- Coverage   70.62%   70.62%   -0.01%     
==========================================
  Files         742      742              
  Lines       32830    32829       -1     
  Branches     3667     3668       +1     
==========================================
- Hits        23186    23185       -1     
  Misses       8930     8930              
  Partials      714      714              

Flag	Coverage Δ
unittests	70.62% <100.00%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
...rc/ImageSharp/Formats/Jpeg/Components/Block8x8F.cs	68.08% <100.00%> (-0.08%)	⬇️

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[JimBobSquarePants]

Lovely @tkp1n I'll have a look tonight. 👍

[antonfirsov]

LGTM. Test output looks good visually.