grid_clear
grid_clear(gd, px, py, nx, ny, bg);
grid_clear(gd, 0, py + ny2, gd->sx, ny - ny2, bg);
grid_clear(gd, 0, sy - ny, gd->sx, ny, bg);
grid_clear(gd, 0, py + ny2, gd->sx, ny - ny2, bg);
grid_clear(gd, px, py, 1, 1, bg);
grid_clear(gd, sx - nx, py, nx, 1, bg);
void grid_clear(struct grid *, u_int, u_int, u_int, u_int, u_int);