Function render_to_rgba_viewport 

pub fn render_to_rgba_viewport(
    list: &DisplayList,
    pixel_w: u32,
    pixel_h: u32,
    dpi: f64,
    icc: Option<&IccCache>,
    no_aa: bool,
) -> Vec<u8> 

Render a display list to RGBA using the viewport code path, with the viewport set to the full page at 1:1 scale.

This exists to audit the viewport pipeline (render_region_prepared_*) against the same baselines the banded PNG path uses. The two paths share render_element and the same display list, so their output should be pixel-identical on a correctly implemented display list. Differences indicate a bug in one of the two culling / epoch / bbox pipelines.

The CLI exposes this as --device viewport-png; the visual test runner uses it to double-cover each sample without maintaining a second baseline.