Add initial MIS implementation

ray-tracing-material/src/microfacet.rs

@@ -57,6 +57,12 @@ impl<R: Rng, D: MicrofacetDistribution + Debug + Sync + Send> Material<R> for Mi
             ray_tracing_core::material::SampleResult::new(w_out, Color::black())
         }
     }
+
+    fn pdf(&self, w_in: Dir3, w_out: Dir3) -> Float {
+        let w_h = (w_in + w_out).normalize();
+
+        self.dist.d(w_h)
+    }
 }
 
 #[derive(Debug)]

ray-tracing-material/src/mirror.rs

@@ -21,4 +21,10 @@ impl<R: Rng> Material<R> for Mirror {
 
         ray_tracing_core::material::SampleResult::new(w_out, self.color)
     }
+
+    fn pdf(&self, w_in: Dir3, w_out: Dir3) -> Float {
+        let _ = w_out;
+        let _ = w_in;
+        0.0
+    }
 }