Normalize constructor parameter names (#1427)

* Normalize constructor parameter names

In order to avoid confusion between constructor parameter names and
member variables, we either used alternate names for the parameters or
prefixed the parameter names with an underscore. For example:

    class foo {
      public:
        foo(int _i, bool semaphore, short bar) : i(_i), flag(semaphore)
        {
            baz = bar;
        }
       private:
         int i;
         bool flag;
         short baz;
    };

However, this is unnecessary. Constructor initializer lists are
unambiguous, as the assigned item always refers to the class member, and
the assigned value is always from the parameter (unless prefixed with
"this->").

Inside the constructor body, parameter names override member variables,
and "this->" can be used to indicate the class member in the case of
name collision.

Thus, the above code can be rewritten as this:

    class foo {
      public:
        foo(int i, bool flag, short baz) : i(i), flag(flag)
        {
            this->baz = baz;
        }
       private:
         int i;
         bool flag;
         short baz;
    };

I've taken advantage of this to clarify names used in constructors
throughout the codebase. Mostly for constructor parameter names,
sometimes for member variable names.

* Refine names from review feedback

books/RayTracingInOneWeekend.html

@@ -1114,7 +1114,7 @@
 
     class sphere : public hittable {
       public:
-        sphere(const point3& _center, double _radius) : center(_center), radius(_radius) {}
+        sphere(const point3& center, double radius) : center(center), radius(radius) {}
 
         bool hit(const ray& r, double ray_tmin, double ray_tmax, hit_record& rec) const override {
             vec3 oc = center - r.origin();
@@ -1551,7 +1551,7 @@
 
         interval() : min(+infinity), max(-infinity) {} // Default interval is empty
 
-        interval(double _min, double _max) : min(_min), max(_max) {}
+        interval(double min, double max) : min(min), max(max) {}
 
         double size() const {
             return max - min;
@@ -2794,8 +2794,8 @@
     class sphere : public hittable {
       public:
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
-        sphere(const point3& _center, double _radius, shared_ptr<material> _material)
-          : center(_center), radius(_radius), mat(_material) {}
+        sphere(const point3& center, double radius, shared_ptr<material> mat)
+          : center(center), radius(radius), mat(mat) {}
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
 
         bool hit(const ray& r, interval ray_t, hit_record& rec) const override {
@@ -2844,7 +2844,7 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ Highlight
     class lambertian : public material {
       public:
-        lambertian(const color& a) : albedo(a) {}
+        lambertian(const color& albedo) : albedo(albedo) {}
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
         const override {
@@ -2896,7 +2896,7 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class lambertian : public material {
       public:
-        lambertian(const color& a) : albedo(a) {}
+        lambertian(const color& albedo) : albedo(albedo) {}
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
         const override {
@@ -2964,7 +2964,7 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ Highlight
     class metal : public material {
       public:
-        metal(const color& a) : albedo(a) {}
+        metal(const color& albedo) : albedo(albedo) {}
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
         const override {
@@ -3099,7 +3099,7 @@
     class metal : public material {
       public:
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
-        metal(const color& a, double f) : albedo(a), fuzz(f < 1 ? f : 1) {}
+        metal(const color& albedo, double fuzz) : albedo(albedo), fuzz(fuzz < 1 ? fuzz : 1) {}
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
@@ -3252,12 +3252,12 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ Highlight
     class dielectric : public material {
       public:
-        dielectric(double index_of_refraction) : ir(index_of_refraction) {}
+        dielectric(double ref_index) : ref_index(ref_index) {}
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
         const override {
             attenuation = color(1.0, 1.0, 1.0);
-            double refraction_ratio = rec.front_face ? (1.0/ir) : ir;
+            double refraction_ratio = rec.front_face ? (1.0/ref_index) : ref_index;
 
             vec3 unit_direction = unit_vector(r_in.direction());
             vec3 refracted = refract(unit_direction, rec.normal, refraction_ratio);
@@ -3267,7 +3267,8 @@
         }
 
       private:
-        double ir; // Index of Refraction
+        double ref_index;  // Refractive index in vacuum or air, or the ratio of the material's
+                           // refractive index over the refractive index of the enclosing media
     };
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     [Listing [dielectric-always-refract]: <kbd>[material.h]</kbd>
@@ -3365,12 +3366,12 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class dielectric : public material {
       public:
-        dielectric(double index_of_refraction) : ir(index_of_refraction) {}
+        dielectric(double ref_index) : ref_index(ref_index) {}
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
         const override {
             attenuation = color(1.0, 1.0, 1.0);
-            double refraction_ratio = rec.front_face ? (1.0/ir) : ir;
+            double refraction_ratio = rec.front_face ? (1.0/ref_index) : ref_index;
 
             vec3 unit_direction = unit_vector(r_in.direction());
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight
@@ -3391,7 +3392,8 @@
         }
 
       private:
-        double ir; // Index of Refraction
+        double ref_index;  // Refractive index in vacuum or air, or the ratio of the material's
+                           // refractive index over the refractive index of the enclosing media
     };
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     [Listing [dielectric-with-refraction]: <kbd>[material.h]</kbd>
@@ -3433,12 +3435,12 @@
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++
     class dielectric : public material {
       public:
-        dielectric(double index_of_refraction) : ir(index_of_refraction) {}
+        dielectric(double ref_index) : ref_index(ref_index) {}
 
         bool scatter(const ray& r_in, const hit_record& rec, color& attenuation, ray& scattered)
         const override {
             attenuation = color(1.0, 1.0, 1.0);
-            double refraction_ratio = rec.front_face ? (1.0/ir) : ir;
+            double refraction_ratio = rec.front_face ? (1.0/ref_index) : ref_index;
 
             vec3 unit_direction = unit_vector(r_in.direction());
             double cos_theta = fmin(dot(-unit_direction, rec.normal), 1.0);
@@ -3459,7 +3461,8 @@
         }
 
       private:
-        double ir; // Index of Refraction
+        double ref_index;  // Refractive index in vacuum or air, or the ratio of the material's
+                           // refractive index over the refractive index of the enclosing media
 
 
     ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ C++ highlight