Lucas Gandel authored Jan 12, 2018 and Mathieu Westphal (Kitware) committed May 24, 2018

Fixing stereo support with Qt5.
Commit 139f787a was introducing QVTKOpenGLWidget to couple a vtkRenderWindow with a QOpenGLWidget.
However, both QVTKOpenGLWidget and its superclass QOpenGLWidget manage a single frame buffer.

QVTKOpenGLWindow derives the QOpenGLWindow class to benefit from an OpenGL context managing multiple frame buffers.
It can be used as a QWidget by calling QWidget::createWindowContainer() on your QVTKOpenGLWindow instance.
QVTKOpenGLWidget has been reimplemented to use thes QVTKOpenGLWindow directly has a widget.
It is recommended to do so.

The old QVTKOpenGLWidget is been moved into QVTKOpenGLSimpleWidget and should not be removed as it support being a native widget,
wich the new implementation does not.

There is a new QVTKOpenGLWidget::isValid() method to check that the widget is ready to render.

It is to be noted that there is a specific implementation of
event handling in order to work around QTBUG-...