Add isotropic constructor taking color argument

Resolves #644

Author: hollasch

books/RayTracingTheNextWeek.html

@@ -3045,7 +3045,7 @@
             constant_medium(shared_ptr<hittable> b, double d, color c)
                 : boundary(b),
                   neg_inv_density(-1/d),
-                  phase_function(make_shared<isotropic>(make_shared<solid_color>(c)))
+                  phase_function(make_shared<isotropic>(c))
                 {}
 
             virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const;
@@ -3071,6 +3071,7 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class isotropic : public material {
         public:
+            isotropic(color c) : albedo(make_shared<solid_color>(c)) {}
             isotropic(shared_ptr<texture> a) : albedo(a) {}
 
             virtual bool scatter(

src/TheNextWeek/constant_medium.h

@@ -29,7 +29,7 @@ class constant_medium : public hittable  {
         constant_medium(shared_ptr<hittable> b, double d, color c)
             : boundary(b),
               neg_inv_density(-1/d),
-              phase_function(make_shared<isotropic>(make_shared<solid_color>(c)))
+              phase_function(make_shared<isotropic>(c))
             {}
 
         virtual bool hit(const ray& r, double t_min, double t_max, hit_record& rec) const;

src/TheNextWeek/material.h

@@ -24,7 +24,7 @@ double schlick(double cosine, double ref_idx) {
 }
 
 
-class material  {
+class material {
     public:
         virtual color emitted(double u, double v, const point3& p) const {
             return color(0,0,0);
@@ -128,11 +128,12 @@ class diffuse_light : public material {
 
 class isotropic : public material {
     public:
+        isotropic(color c) : albedo(make_shared<solid_color>(c)) {}
         isotropic(shared_ptr<texture> a) : albedo(a) {}
 
         virtual bool scatter(
             const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered
-        ) const  {
+        ) const {
             scattered = ray(rec.p, random_in_unit_sphere(), r_in.time());
             attenuation = albedo->value(rec.u, rec.v, rec.p);
             return true;

src/TheRestOfYourLife/material.h

@@ -156,6 +156,7 @@ class diffuse_light : public material {
 
 class isotropic : public material {
     public:
+        isotropic(color c) : albedo(make_shared<solid_color>(c)) {}
         isotropic(shared_ptr<texture> a) : albedo(a) {}
 
         virtual bool scatter(