D29247: [effects/showfps] Calculate "correct" FPS

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REVISION SUMMARY
  cca0e15b455ba6 <https://phabricator.kde.org/R108:cca0e15b455ba606043c720479490794d8d86309> broke code that calculates the value of the fps counter.
  
  The frames field stores timestamps of previous frames, not time between
  each two consecutive frames.
  
  This change doesn't attempt to make the show fps compute more accurate
  performance metrics, e.g. how much times it takes to execute rendering
  commands on the GPU, etc.

TEST PLAN
  The Show FPS effect displays more sensible values.

REPOSITORY
  R108 KWin

BRANCH
  showfps-correct-timestamp

REVISION DETAIL
  https://phabricator.kde.org/D29247

AFFECTED FILES
  effects/showfps/showfps.cpp
  effects/showfps/showfps.h

To: zzag, #kwin
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