Draw a box on the virtual display
void SLsmg_draw_box (int r, int c, unsigned int dr, unsigned int dc)
SLsmg_draw_box uses the SLsmg_draw_hline and
SLsmg_draw_vline functions to draw a rectangular box on the
virtual display. The box's upper left corner is placed at row
r and column c. The width and length of the box is
specified by dc and dr, respectively.
SLsmg_draw_vline, SLsmg_draw_hline, SLsmg_draw_object