Break when minY >= bottom

[JimBobSquarePants]

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Description

The ParallelHelper calculation did not correctly handle certain edge cases when the height of a vertical step exceeded the height/number of steps resulting in RowInterval.MinY greater than RowInterval.MaxY.

This fix returns early in the parallel action when this case is detected. Fixes #1105

[codecov]

Codecov Report

Merging #1106 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1106      +/-   ##
==========================================
+ Coverage   82.22%   82.23%   +<.01%     
==========================================
  Files         702      702              
  Lines       29159    29175      +16     
  Branches     3296     3298       +2     
==========================================
+ Hits        23977    23993      +16     
  Misses       4491     4491              
  Partials      691      691

Flag	Coverage Δ
#unittests	82.23% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
...mageSharp/Advanced/ParallelUtils/ParallelHelper.cs	100% <100%> (ø)	⬆️

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[antonfirsov]

LGTM. It took me a while to figure out what I was doing here originally 😄

Pushed a few more tests to really make sure it works on supercomputers.