camera: cache inverse tranform for huge speed up on ray_for_pixel.

WIP parallel render function.

rtchallenge/examples/eoc7.rs

@@ -25,7 +25,7 @@ fn main() -> Result<()> {
     let from = Tuple::point(0., 1.5, -5.);
     let to = Tuple::point(0., 1., 0.);
     let up = Tuple::point(0., 1., 0.);
-    camera.transform = view_transform(from, to, up);
+    camera.set_transform(view_transform(from, to, up));
 
     let mut floor = Sphere::default();
     floor.set_transform(Matrix4x4::scaling(10., 0.01, 10.));