Function fill_path

fn fill_path(
    path: &Path,
    w: usize,
    h: usize,
    cov: &mut Vec<f32>,
    edges: &mut Vec<Edge>,
    active: &mut Vec<usize>,
    xs: &mut Vec<(f32, i32)>,
) -> Option<(usize, usize, usize, usize)>

Scanline fill with 4x vertical sub-sampling and exact horizontal coverage. Writes cov (zeroing only the band it touches) and returns that band as (x0, y0, x1, y1).