Function render_view_lines 

pub fn render_view_lines<'a>(
    meshes: impl IntoIterator<Item = &'a Mesh>,
    lines: &[([V3; 2], V3)],
    w: u32,
    h: u32,
    vp: &M4,
    to_eye: V3,
    default_color: V3,
) -> RgbaImage

render_view with line segments drawn over the faces, each in its colour, about a pixel wide and hidden where a face is in front of them (the edges, grid and axes the viewer shows).