M_AddToBox
( fixed_t*	box,
  fixed_t	x,
  fixed_t	y )
{
    if (x<box[BOXLEFT])
	box[BOXLEFT] = x;
    else if (x>box[BOXRIGHT])
	box[BOXRIGHT] = x;
    if (y<box[BOXBOTTOM])
	box[BOXBOTTOM] = y;
    else if (y>box[BOXTOP])
	box[BOXTOP] = y;
}