D29705: Correctly align natural layout in present windows

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REVISION SUMMARY
  Present windows works as follows:
  
  - It moves all windows about until nothing is overlapping with any
  
  other window.
  
  - This doesn't resize anything so ultimately we end up with a new
  
  co-ordinate space that's bigger than the screen depending on the amount
  of overlap.
  
  - We then render this whole view transformed to the screen
  
  The rectangle "bounds" is in overviewpixels, with "scale" being the
  ratio to convert to screen pixels.
  
  When adjusting the new bounds there's an attempt to centre align things.
  As bounds is in "overviewpixels" we multiply references to the previous
  bounds by scale, and divide everything through at the end. bounds.x/y
  were missed.
  
  This is mostly unoticable except on massive super-ultra-wide monitors
  which will otherwise have a tendency to shift to the left.

TEST PLAN
  Kai created a whole new test framework for this code that copy pasted 
  this algorithm then showed mock windows as rectangles

REPOSITORY
  R108 KWin

BRANCH
  master

REVISION DETAIL
  https://phabricator.kde.org/D29705

AFFECTED FILES
  effects/presentwindows/presentwindows.cpp

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