Function rasterize_at 

pub fn rasterize_at(
    ops: &[PathOp],
    scale: f32,
    off: Vec2,
    w: u32,
    h: u32,
    paint: MaskPaint,
) -> Vec<u8> 

rasterize with the box’s origin off physical px into the mask: what a turning path’s mask is drawn with, its pivot at the mask’s centre.