pub fn square_to_quad(q: &[[f32; 2]; 4]) -> [[f32; 3]; 3]Expand description
Homography mapping the unit square (0,0)(1,0)(1,1)(0,1) onto q (Heckbert).
ponytail: copied from compose::square_to_quad (private there) so the GPU geometry cannot drift
from the CPU one — make that pair pub(crate) and this goes away.