Function render_png_padded 

pub fn render_png_padded(
    tree: &Tree,
    px: u32,
    pad: f32,
) -> Result<Vec<u8>, String>

Like render_png, with pad (a fraction of the edge, e.g. 0.1 = 10 %) of transparent margin on every side — the macOS icon convention (art inset on the 1024 canvas).