newt
startangle = inputAngle;
normalangle = normalAngle;
var vx = Math.cos(startangle);
var vy = Math.sin(startangle);
var nx = Math.cos(normalangle);
var ny = Math.sin(normalangle);
var v_dot_n = vx * nx + vy * ny;
var rx = vx - 2 * v_dot_n * nx;
var ry = vy - 2 * v_dot_n * ny;
var ra = cr.angleTo(0, 0, rx, ry);