D25907: [platforms/drm] Introduce Gl post-process output rotations

romangg created this revision.
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REVISION SUMMARY
  In case the hardware is not able to rotate the output for the configured
  rotation value do this rotation in a post-process step.
  
  For that rendering the current view into a separate framebuffer bound to a
  texture that then gets sampled to the default framebuffer in an additional
  rendering pass through a simple shader rotating it.
  
  This allows us to leave the Effects system and internal model-view-projection
  matrix untouched. The rotation in the post-processing step is isolated.

TEST PLAN
  With KScreen all rotations work.

REPOSITORY
  R108 KWin

BRANCH
  post-process-rotation

REVISION DETAIL
  https://phabricator.kde.org/D25907

AFFECTED FILES
  plugins/platforms/drm/drm_output.cpp
  plugins/platforms/drm/egl_gbm_backend.cpp
  plugins/platforms/drm/egl_gbm_backend.h

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