fg_add_stroke
fg_add_stroke(sc, &pspans_x[0], &pspans_y[0]);
fg_add_stroke(sc, &spans[X][i], &spans[Y][j]);
fg_add_stroke(sc, &spans[X][i], &spans[Y][j]);
static void fg_add_stroke(struct atp_softc *, const fg_pspan *, const fg_pspan *);