message: Fix angle calculation
  TODO: do this w/o using trig functions..
diff: @@ -28,7 +28,7 @@ public:
  {
  U sgn = select(dot(n, h) > U(0.0), U(1.0), U(0.0));
  - U theta_h = dot(n, h);
  + U theta_h = acos(dot(n, h));
  U cos4 = cos(theta_h) * cos(theta_h) * cos(theta_h) * cos(theta_h);
  U tan2 = tan(theta_h) * tan(theta_h);
  @@ -42,7 +42,7 @@ public:
  {
  U sgn = select(dot(w, h) / dot(w, n) > U(0.0), U(1.0), U(0.0));
  - U theta_v = dot(n, w);
  + U theta_v = acos(dot(n, w));
  U tan2 = tan(theta_v) * tan(theta_v);
  return sgn * U(2.0)
  