Fix #1006 Resize from large source to small thumbnail produces deformed top pixel rows

[JimBobSquarePants]

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Description

Turns out this had been fixed before but somehow regressed. I've re-added the fix and added a test reference image to ensure it does not regress again.

@antonfirsov When you have some time I'd like to chat to you about this comment I discovered during investigation.

ImageSharp/src/ImageSharp/Processing/Processors/Transforms/Resize/ResizeProcessor{TPixel}.cs

Line 133 in 1017121

// To reintroduce parallel processing, we to launch multiple workers

Amazing work getting the performance you have done in series. Do you think we should reintroduce parallelism here?

[codecov]

Codecov Report

Merging #1060 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1060   +/-   ##
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  Coverage   89.94%   89.94%           
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  Files        1131     1131           
  Lines       50619    50619           
  Branches     3715     3715           
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  Hits        45528    45528           
  Misses       4335     4335           
  Partials      756      756

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[tocsoft]

lgtm 👍