Implement RandomSampler

[wahn]

The example scene assets/scenes/veach-mis/mis.pbrt uses a RandomSampler:

> grep Sampler assets/scenes/veach-mis/mis.pbrt
Sampler "random" "integer pixelsamples" [128]

Right now we have three implementors of the trait Sampler:

> rg -trust "impl Sampler for"
src/samplers/sobol.rs
90:impl Sampler for SobolSampler {

src/samplers/zerotwosequence.rs
96:impl Sampler for ZeroTwoSequenceSampler {

src/samplers/halton.rs
178:impl Sampler for HaltonSampler {

In C++ the class RandomSampler ist directly inherited from Sampler:

class RandomSampler : public Sampler {
  public:
    RandomSampler(int ns, int seed = 0);
    void StartPixel(const Point2i &);
    Float Get1D();
    Point2f Get2D();
    std::unique_ptr<Sampler> Clone(int seed);

  private:
    RNG rng;
};

[wahn]

This is how the C++ version renders:

[wahn]

Here is the current (commit a341658) Rust version:

> imf_diff -d -f pbrt_rust.exr ~/tmp/pbrt/mis.exr diff.jpg
differing pixels:	 12.645% (125859 of 995328)
average difference:	  9.304%
maximum difference:	 66.128%
Summary: Many pixels differ strongly.
== "pbrt_rust.exr" and "/usr/people/jan/tmp/pbrt/mis.exr" are different

Let's investigate where that difference between the C++ and Rust version of PBRT comes from ...

[wahn]

Commit db66de4 solves the difference between C++ and Rust by changing the order of RNG calls:

@@ -68,9 +68,9 @@ impl Sampler for RandomSampler {                                                                                          
     fn get_2d(&mut self) -> Point2f {                                                                                                      
         // TODO: ProfilePhase _(Prof::GetSample);                                                                                          
         assert!(self.current_pixel_sample_index < self.samples_per_pixel);                                                                 
-        // C++: call y first                                                                                                               
-        let y = self.rng.uniform_float();                                                                                                  
+        // C++: call x first                                                                                                               
         let x = self.rng.uniform_float();                                                                                                  
+        let y = self.rng.uniform_float();                                                                                                  
         Point2f {                                                                                                                          
             x: x,                                                                                                                          
             y: y,                                                                                                                          

> imf_diff -d -f pbrt_rust.exr mis.exr diff.jpg
differing pixels:	  0.000% (2 of 995328)
average difference:	  1.593%
maximum difference:	  1.698%
Summary: 2 pixels differ slightly.
== "pbrt_rust.exr" and "mis.exr" are similar