When using a Simple Refresh or a Smart Refresh windows, the program may
receive refresh events, informing it to update the display.  See the above
discussion for information on when refresh events are sent.

A message of the class IDCMP_REFRESHWINDOW arrives at the IDCMP, informing
the program of the need to update the display.  The program must take some
action when it receives a refresh event, even if it is just the acceptable
minimum action described below.

On receiving a refresh event, BeginRefresh() must be called, then the
program should redraw its display, and, finally, call EndRefresh().  The
minimum required action is to call the BeginRefresh()/EndRefresh() pair.
This allows Intuition and the Layers library keep things sorted and
organized.