D19540: Verify that there is no scene windows when Scene is about to be destroyed

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REVISION SUMMARY
  When ~Scene is called, it's not guaranteed that backend is still valid
  or that there is current opengl context. So, generally speaking,
  deleting scene windows in ~Scene is a bad idea.
  
  It's safe not to delete scene windows in ~Scene because current
  implementation of Compositor::finish ensures that they don't last longer
  than the scene.

REPOSITORY
  R108 KWin

BRANCH
  assert-empty-windows

REVISION DETAIL
  https://phabricator.kde.org/D19540

AFFECTED FILES
  scene.cpp

To: zzag, #kwin
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