WIP: Add QVTKOpenGLWidget2|QVTKOpenGLWindow class based on QOpenGLWindow
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. You can also use QVTKOpenGLWidget2 that provides a direct wrapper around QVTKOpenGLWindow, to use it as a QWidget.
As of now, QVTKOpenGLWidget2 implementation is adapted from QVTKWidget2 (which supports stereo rendering with Qt5 but subclasses deprecated QGLWidget). When this new class layout gets validated, the following functionalities from QVTKWidget2 should be added to QVTKOpenGLWidget2 to finish the transition: -TDx support -Support for ContextMenu|Drag|Drop events