Function render_region_prepared_parallel_cancellable 

pub fn render_region_prepared_parallel_cancellable(
    list: &DisplayList,
    prepared: &PreparedDisplayList,
    vp_x: f64,
    vp_y: f64,
    vp_w: f64,
    vp_h: f64,
    pixel_w: u32,
    pixel_h: u32,
    dpi: f64,
    icc: Option<&IccCache>,
    image_cache: Option<&ImageCache>,
    no_aa: bool,
    cancelled: &AtomicBool,
) -> Option<Vec<u8>>

Like render_region_prepared_parallel() but checks a cancellation flag between band chunks. Returns None if cancelled.