QVTKOpenGLWidget: vtkGenericOpenGLRenderWindow::GetScreenSize() should return something other than NULL
QVTKOpenGLWidget requires using vtkGenericOpenGLRenderWindow. Currently, vtkGenericOpenGLRenderWindow::GetScreenSize() unconditionally returns NULL. This breaks client code like https://github.com/Slicer/Slicer/blob/4e873be/Libs/MRML/DisplayableManager/vtkMRMLCrosshairDisplayableManager.cxx#L318.
QVTKOpenGLWidget could be responsible for determining the size of the its screen--using Qt--and setting that screen size on its vtkGenericOpenGLRenderWindow.