Function render_region_prepared_parallel_with_progress 

pub fn render_region_prepared_parallel_with_progress(
    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,
    progress: &AtomicU32,
) -> Vec<u8> 

Like render_region_prepared_parallel() but with an atomic progress counter.

The counter is incremented after each chunk of bands completes. The total number of bands is returned alongside the counter via viewport_band_count().