D25300: [platforms/x11/standalone] Use intel swap events by default

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REVISION SUMMARY
  Activate intel swap events by default if available. They were hidden behind an
  environmental variable because of some critical release blocking issue years
  ago.
  
  Manual testing indicates that there are no issues anymore with this extension.
  Since it allows us to use swap events with MESA drivers for optimized repaints
  enable swap events by default again.
  
  For now leave a modified environment variable to switch back to using no swap
  events easily.
  
  CCBUG: 342582

TEST PLAN
  i915

REPOSITORY
  R108 KWin

BRANCH
  default-intel-swap

REVISION DETAIL
  https://phabricator.kde.org/D25300

AFFECTED FILES
  plugins/platforms/x11/standalone/glxbackend.cpp

To: romangg, #kwin
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