Now we come to the event handlers, often called callbacks. The first of these is the reshape callback cbReshape().
Actually, graphics programs do not need to specify this handler; one is provided automatically by OpenGL. It is useful to supply one, however, when one wants to avoid distortions that can result when the window is reshaped or resized.
We will skip over most of this function, but notice the two input parameters width and height. These are supplied by the system and contain the current dimensions of the window.