Make stereo modes work again
Be aware that stereo rendering based on SoRenderManager does not allow background colors as the opengl buffers are cleared before rendering. As FreeCAD draws the background before the standart coin rendering the background gets erased again.
Prevent QPainter buffer swap
As the QGraphicsScene overlay does not work for all cards/drivers we disable the possible buffer swap of the qpainter, as this could be a potential source of the flickering. If the painter swaps buffers and we do it again the user will see first the correct picture (first swap) and then something uninitalized (second swap)