D22150: [plugins/qpa] Implement native offscreen surface

zzag created this revision.
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REVISION SUMMARY
  Depending on whether the underlying platform supports offscreen surfaces,
  QOffscreenSurface may create an invisible QWindow. In our case that's the
  case, for each offscreen surface a native window is created. This may
  lead to some funky results related to window decorations, see bug 407612.
  
  There are several ways to implement offscreen surfaces - either use pbuffers
  or utilize a surfaceless context extension. For the sake of simplicity
  this change sticks with pbuffers, but it's a good idea to support both
  methods.
  
  CCBUG: 407612

REPOSITORY
  R108 KWin

BRANCH
  qpa-offscreen-surface

REVISION DETAIL
  https://phabricator.kde.org/D22150

AFFECTED FILES
  plugins/qpa/CMakeLists.txt
  plugins/qpa/abstractplatformcontext.cpp
  plugins/qpa/egl_helpers.cpp
  plugins/qpa/egl_helpers.h
  plugins/qpa/integration.cpp
  plugins/qpa/integration.h
  plugins/qpa/offscreen_surface.cpp
  plugins/qpa/offscreen_surface.h
  plugins/qpa/sharingplatformcontext.cpp

To: zzag, #kwin
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