Function dg_toggle_rect 

pub fn dg_toggle_rect(n: &DNode, bar: f32) -> [f32; 4]

the world box x, y, w, h of a DN_TOGGLE node’s collapse button: the square of the title bar’s height bar at its right end (a DN_FRAME node), else a square of that side (at most the node’s) in the top right corner; apps paint the button there