Bouncing is an example of how you can extend the graphics engine. It
subclasses GLevelCanvas and GTileForm to allow tiles to be drawn both below
and above the sprites; and subclasses GSprite and GPixels to change the
color of a single (grey) ball image. You can move the spinning ball with
the arrow keys, and collect the bouncing balls.
You can type "debug" to see exactly how much of the pixel buffer is updated each frame. buffer is updated each frame.