Filling the same circle with a different color leaves an outline

[euclio]

With the following code:

use std::f32::consts::TAU;

use raqote::*;

fn main() {
    let mut dt = DrawTarget::new(400, 400);

    let black = SolidSource::from_unpremultiplied_argb(0xFF, 0x00, 0x00, 0x00);
    dt.clear(black);

    let mut pb = PathBuilder::new();
    pb.arc(200., 200., 100., 0., TAU);
    let path = pb.finish();

    dt.fill(
        &path,
        &Source::Solid(SolidSource::from_unpremultiplied_argb(0xFF, 0xFF, 0xFF, 0)),
        &DrawOptions::new(),
    );

    dt.fill(
        &path,
        &Source::Solid(black),
        &DrawOptions::new(),
    );

    dt.write_png("out.png").unwrap();
}

I would expect the result to be a completely black image.

However, a thin outline is left: