If set to true the render signal will be emitted every time the widget draws.
This is the default and is useful if drawing the widget is faster.
If set to false the data from previous rendering is kept around and will be used for drawing the widget the next time, unless the window is resized. In order to force a rendering [[email protected]_render] must be called. This mode is useful when the scene changes seldom, but takes a long time to redraw.