Add initial MIS implementation

ray-tracing-scene/src/triangle_bvh.rs

@@ -329,13 +329,16 @@ impl<R: Rng> Scene<R> for TriangleBVH<R> {
         let triangle = self.triangles[i as usize];
         let material = &self.materials[triangle.material as usize];
 
-        let n = triangle_normal(self.get_vertices_from_index(i)).normalize();
+        let n = triangle_normal(self.get_vertices_from_index(i));
+
+        let area = n.length() * 0.5;
 
         Some(Intersection::new(
             t,
-            n,
+            n.normalize(),
             material.material.as_deref(),
             material.light.as_deref(),
+            1.0 / ((self.lights.len() as Float) * area),
         ))
     }