bvh_node: don't modify source object list

Instead of reordering the vector of scene objects passed to the
`bvh_node` constructor, it now creates a local copy instead. For some
reason, this is actually slightly faster.

Resolves #701

Author: hollasch

CHANGELOG.md

@@ -12,13 +12,19 @@ Change Log -- Ray Tracing in One Weekend
   - Fix: Listings 33, 39: Add  consistent function signature for `trilinear_interp` (#722)
 
 ### _The Next Week_
+  - Change: `bvh_node` no longer reorders the source vector of scene objects; uses local copy
+    instead (#701)
   - Delete: remove unused u,v,w variables in initial `perlin::noise()` function (#684)
   - Fix: Listing 15: In `bvh.h`, add missing `hittable_list.h` include (#690)
   - Fix: Listing 33, 34, 38: Change implicit casts to explicit ones (#692)
   - Fix: Listing 40: Change `perlin.h` in the caption to `texture.h` (#698)
   - Fix: Listing 70: Add missing `bvh.h` (#694)
   - Fix: Listing 70 and `main.cc`: Change a fuzz value of a metal sphere to 1.0 which is the maximum value (#694)
 
+### _The Next Week_
+  - Change: `bvh_node` no longer reorders the source vector of scene objects; uses local copy
+    instead (#701)
+
 
 ----------------------------------------------------------------------------------------------------
 # v3.2.0 (2020-07-18)
@@ -59,8 +65,6 @@ significant change and improvement. We're hoping that changes to books one and t
 but that's never worked out for us before. Ah, dreams.
 
 ### Common
-  - Bug: Found a bug in book 3 source `isotropic::scatter()` method. Commented out, using default
-    (as it was previously). (#669)
   - Delete: vestigial `vec3::write_color()` method (now in color.h)
   - Change: All images and figures renamed to follow more logical convention, using the following
     pattern: `{fig,img}-<book>.<sequence>-<title>.<filetype>` (#495)
@@ -76,6 +80,8 @@ but that's never worked out for us before. Ah, dreams.
     because it already caught an existing bug in _The Rest of Your Life_ source. This change
     includes commenting out the book 3 `isotropic::scatter()` method, which was accidentally ignored
     anyway. (#639, #669)
+  - Fix: Found a bug in book 3 source `isotropic::scatter()` method. Commented out, using default
+    (as it was previously). (#669)
   - New: each book gets a section of recommended citation examples (#500)
 
 ### _In One Weekend_

books/RayTracingTheNextWeek.html

@@ -861,6 +861,7 @@
     #define BVH_H
 
     #include "rtweekend.h"
+
     #include "hittable.h"
     #include "hittable_list.h"
 
@@ -869,12 +870,12 @@
         public:
             bvh_node();
 
-            bvh_node(hittable_list& list, double time0, double time1)
+            bvh_node(const hittable_list& list, double time0, double time1)
                 : bvh_node(list.objects, 0, list.objects.size(), time0, time1)
             {}
 
             bvh_node(
-                std::vector<shared_ptr<hittable>>& objects,
+                const std::vector<shared_ptr<hittable>>& src_objects,
                 size_t start, size_t end, double time0, double time1);
 
             virtual bool hit(
@@ -951,6 +952,8 @@
         std::vector<shared_ptr<hittable>>& objects,
         size_t start, size_t end, double time0, double time1
     ) {
+        auto objects = src_objects; // Create a modifiable array of the source scene objects
+
         int axis = random_int(0,2);
         auto comparator = (axis == 0) ? box_x_compare
                         : (axis == 1) ? box_y_compare

src/TheNextWeek/bvh.h

@@ -23,12 +23,12 @@ class bvh_node : public hittable  {
     public:
         bvh_node();
 
-        bvh_node(hittable_list& list, double time0, double time1)
+        bvh_node(const hittable_list& list, double time0, double time1)
             : bvh_node(list.objects, 0, list.objects.size(), time0, time1)
         {}
 
         bvh_node(
-            std::vector<shared_ptr<hittable>>& objects,
+            const std::vector<shared_ptr<hittable>>& src_objects,
             size_t start, size_t end, double time0, double time1);
 
         virtual bool hit(
@@ -68,9 +68,11 @@ bool box_z_compare (const shared_ptr<hittable> a, const shared_ptr<hittable> b)
 
 
 bvh_node::bvh_node(
-    std::vector<shared_ptr<hittable>>& objects,
+    const std::vector<shared_ptr<hittable>>& src_objects,
     size_t start, size_t end, double time0, double time1
 ) {
+    auto objects = src_objects; // Create a modifiable array of the source scene objects
+
     int axis = random_int(0,2);
     auto comparator = (axis == 0) ? box_x_compare
                     : (axis == 1) ? box_y_compare

src/TheRestOfYourLife/bvh.h

@@ -14,6 +14,7 @@
 #include "rtweekend.h"
 
 #include "hittable.h"
+#include "hittable_list.h"
 
 #include <algorithm>
 
@@ -22,12 +23,12 @@ class bvh_node : public hittable  {
     public:
         bvh_node();
 
-        bvh_node(hittable_list& list, double time0, double time1)
+        bvh_node(const hittable_list& list, double time0, double time1)
             : bvh_node(list.objects, 0, list.objects.size(), time0, time1)
         {}
 
         bvh_node(
-            std::vector<shared_ptr<hittable>>& objects,
+            const std::vector<shared_ptr<hittable>>& src_objects,
             size_t start, size_t end, double time0, double time1);
 
         virtual bool hit(
@@ -67,9 +68,11 @@ bool box_z_compare (const shared_ptr<hittable> a, const shared_ptr<hittable> b)
 
 
 bvh_node::bvh_node(
-    std::vector<shared_ptr<hittable>>& objects,
+    const std::vector<shared_ptr<hittable>>& src_objects,
     size_t start, size_t end, double time0, double time1
 ) {
+    auto objects = src_objects; // Create a modifiable array of the source scene objects
+
     int axis = random_int(0,2);
     auto comparator = (axis == 0) ? box_x_compare
                     : (axis == 1) ? box_y_compare