The HAVE_XCOMPOSITE_OVERLAY define had not been moved to the new
overlaywindow.cpp file causing the OverlayWindow to not be created
at all. In fact after moving the define to the new file the code
did not even compile as that code parts still used m_overlayWindow
instead of m_window.
The regression would have been easy to spot when testing with all
backends as the GLES/EGL backend still requires the overlay window.
CCMAIL: a.arlt@stud.uni-heidelberg.de
All the functionality of Overlay Window is moved to its own class
OverlayWindow. It is created and owned by class Scene, since almost
all function calls are called from this class.
REVIEW: 101866
