D18366: Fix flickering with Qt 5.12

volkov created this revision.
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REVISION SUMMARY
  KWin expects Qt to produce xcb_glx_buffer_swap_complete_event_t
  events with DRI2, but Qt 5.12 initializes GLX integration only
  when required, e.g. when a window with OpenGL support is
  created or when availability of OpenGL is checked.
  
  So use QOpenGLContext::supportsThreadedOpenGL() to force
  initialization of GLX integration in the Qt's xcb plugin.
  
  https://bugzilla.opensuse.org/show_bug.cgi?id=1120090

REPOSITORY
  R108 KWin

BRANCH
  Plasma/5.15

REVISION DETAIL
  https://phabricator.kde.org/D18366

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

To: volkov
Cc: kwin, jraleigh, GB_2, mkulinski, ragreen, jackyalcine, Pitel, iodelay, bwowk, ZrenBot, ngraham, lesliezhai, ali-mohamed, hardening, jensreuterberg, abetts, sebas, apol, mart