camera: make supersampling configurable, wire it up in eoc8.

rtchallenge/examples/eoc8.rs

@@ -22,6 +22,10 @@ use rtchallenge::{
 struct Opt {
     #[structopt(long, default_value = "rayon")]
     render_strategy: RenderStrategy,
+    /// Number of samples per pixel.  0 renders from the center of the pixel, 1 or more samples N
+    /// times randomly across the pixel.
+    #[structopt(short, long, default_value = "0")]
+    samples: usize,
 }
 
 fn main() -> Result<()> {
@@ -38,6 +42,7 @@ fn main() -> Result<()> {
     let up = Tuple::point(0., 1., 0.);
     camera.set_transform(view_transform(from, to, up));
     camera.render_strategy = opt.render_strategy;
+    camera.samples_per_pixel = opt.samples;
 
     let mut floor = Sphere::default();
     floor.set_transform(Matrix4x4::scaling(10., 0.01, 10.));