Function compose_page 

pub fn compose_page(
    page_w: u32,
    page_h: u32,
    items: &[PagePlacement<'_>],
) -> Vec<u8> 

Composite items onto a white page_w × page_h page, returning tightly packed RGBA8. Each item’s snapshot is nearest-neighbor scaled into its destination rectangle (clipped to the page) and src-over composited on white. This is the no-dep substitute for silx’s QPrinter scene render — the analogue of its print-to-file path.