---------------------------------------------------------------------------
  vzoomwindow                                                           WNDWC
  ---------------------------------------------------------------------------
  Function       Toggles top virtual window between full screen size and
                 original size.
  Syntax         void vzoomwindow(void);
  Remarks        If the window is less than full size, the window is zoomed
                 to full size and centered if necessary.  Executing the
                 function again will restore the size and location of this
                 window.  If NOMOVEMODE or PERMMODE has been set for this
                 window, the function does nothing.  Shadows are fully
                 supported.  Margins are respected.
  Return value   None.
  Heap           Maximum requirement is full screen size plus the underlay
                 size.
  Upon return    Forced to write to CRT.