Function render_region_single_band 

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

Render a single horizontal band of a viewport region.

This is the per-band counterpart to render_region_prepared(). The caller loops over band_idx in 0..num_bands, collecting RGBA strips that tile vertically to form the full viewport image.

Returns RGBA pixel data for actual_h rows (may be less than band_h for the last band).