stroke round_rect line width bug

[goaq]

When drawing round_rect, the line width cannot be set to 1px

    plutovg_save(pluto);
    plutovg_round_rect(pluto, 10, 10, 580, 50, 5, 5);
    plutovg_set_rgb(pluto, 0, 0, 1);
    plutovg_set_line_width(pluto, 1);
    plutovg_stroke_preserve(pluto);
    plutovg_stroke(pluto);
    plutovg_restore(pluto);