Verify that there is no scene windows when Scene is about to be destroyed
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 okay not to delete scene windows in ~Scene because current
implementation of Compositor::finish ensures that they don't last longer
than the scene.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Differential Revision: https://phabricator.kde.org/D19540