pub fn map_unit_square(rect: Rect) -> Affine {
        Affine([rect.width(), 0., 0., rect.height(), rect.x0, rect.y0])
    }
Creates an affine transformation that takes the unit square to the given rectangle.

Useful when you want to draw into the unit square but have your output fill any rectangle.
In this case push the `Affine` onto the transform stack.
common/src/geometry.rs