Improve Pixelate filter scale

[SuslikV]

Fixes: #506

[codecov-io]

Codecov Report

Merging #507 into develop will decrease coverage by 0.01%.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           develop     #507      +/-   ##
===========================================
- Coverage    48.25%   48.23%   -0.02%     
===========================================
  Files          128      128              
  Lines         9954     9957       +3     
===========================================
  Hits          4803     4803              
- Misses        5151     5154       +3     

Impacted Files	Coverage Δ
src/effects/Pixelate.cpp	0.00% <0.00%> (ø)

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[ferdnyc]

This looks pretty good to me, though I haven't tested it very thoroughly yet.

Sorry, I got bogged down when I started testing with the discovery that I screwed up the implementation of the effect borders, which seem to be pretty much broken on two sides. (Top, for sure.)

I'm having a bear of a time getting it to work right, QRect and QPainter seem to be having these weird, nondeterministic interactions. Decided to leave it alone for now and come back to it tomorrow.