Nondestructively superimposes win1 on win2. The function writes
    the contents of win1 that will fit onto win2 beginning at the
    starting coordinates of both windows. Blanks on win1 leave
    the contents of the corresponding space on win2 unaltered. The
    overlay function copies as much of a window's box as possible.

    Format

      #include  <curses.h>

      int overlay  (WINDOW *win1, WINDOW *win2);