Edges of shapes are not rendered well

[Long0x0]

Black rectangle expected to be invisible:

    let black = vger.color_paint(Color{r: 0.0, g: 0.0, b: 0.0, a: 1.0});
    vger.fill_rect(euclid::rect(100.0, 100.0, 100.0, 100.0), 10.0, black);

(5x)

Two rectangles expected to be seamless：

    let cyan = vger.color_paint(Color::CYAN);
    vger.fill_rect(euclid::rect(100.0, 100.0, 100.0, 100.0), 0.0, cyan);
    vger.fill_rect(euclid::rect(200.0, 100.0, 100.0, 100.0), 0.0, cyan);

(5x)

I guess these artifacts are caused by anti-aliasing.