Add generalized bvh for pbrt

ray-tracing-core/src/aabb.rs

@@ -49,6 +49,14 @@ impl AABB {
         self.max
     }
 
+    pub fn center(self) -> Pos3 {
+        Pos3::new(
+            (self.min.x + self.max.x) * 0.5,
+            (self.min.y + self.max.y) * 0.5,
+            (self.min.z + self.max.z) * 0.5,
+        )
+    }
+
     pub fn contains_point(self, pos: Pos3) -> bool {
         self.min.x <= pos.x
             && pos.x <= self.max.x