This can happen in some graphics drivers if the focus is changed to a window not being the demo window. It's due
to the rendering we do for calculating the Ambient Occlussion in the preprocess. That's why only fullscreen mode is
allowed in the final version of the intro.
Some zbuffer resolution problems in a well known graphics card's type gave this banding artifact if no depth bias was added to
the z-comparisong in the pixel shader responsible for the shadows.
