Include hittable.h from material.h

The prior version used a forward declaration of `hit_record`, which was
insufficient since hittable.h lines referenced members of `hit_record`,
and was unnecessary because hittable.h was included before material.h in
practice.

For this specific code, the inclusion is unnecessary, but is proper
form.

Resolves #1608

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -2791,7 +2791,7 @@
 
     #include "rtweekend.h"
 
-    class hit_record;
+    #include "hittable.h"
 
     class material {
       public:

books/RayTracingTheNextWeek.html

@@ -1346,13 +1346,11 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     #include "rtweekend.h"
 
-
+    #include "hittable.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
     #include "texture.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
 
-    class hit_record;
-
     ...
 
     class lambertian : public material {

books/RayTracingTheRestOfYourLife.html

@@ -2286,14 +2286,12 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     #include "rtweekend.h"
 
-
+    #include "hittable.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
     #include "onb.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     #include "texture.h"
 
-    class hit_record;
-
     class material {
       public:
         ...
@@ -3329,11 +3327,10 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     #include "rtweekend.h"
 
+    #include "hittable.h"
     #include "onb.h"
     #include "texture.h"
 
-    class hit_record;
-
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
     class scatter_record {
@@ -3370,15 +3367,14 @@
     #include "rtweekend.h"
 
 
+    #include "hittable.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ delete
     #include "onb.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
     #include "pdf.h"
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     #include "texture.h"
 
-    class hit_record;
-
     ...
 
     class lambertian : public material {

src/InOneWeekend/material.h

@@ -13,7 +13,7 @@
 
 #include "rtweekend.h"
 
-class hit_record;
+#include "hittable.h"
 
 
 class material {

src/TheNextWeek/material.h

@@ -13,10 +13,9 @@
 
 #include "rtweekend.h"
 
+#include "hittable.h"
 #include "texture.h"
 
-class hit_record;
-
 
 class material {
   public:

src/TheRestOfYourLife/material.h

@@ -13,11 +13,10 @@
 
 #include "rtweekend.h"
 
+#include "hittable.h"
 #include "pdf.h"
 #include "texture.h"
 
-class hit_record;
-
 
 class scatter_record {
   public: