Clarify intended type of double value

Resolves #1249

Author: hollasch

books/RayTracingInOneWeekend.html

@@ -120,7 +120,7 @@
             for (int i = 0; i < image_width; i++) {
                 auto r = double(i) / (image_width-1);
                 auto g = double(j) / (image_height-1);
-                auto b = 0;
+                auto b = 0.0;
 
                 int ir = int(255.999 * r);
                 int ig = int(255.999 * g);
@@ -259,7 +259,7 @@
             for (int i = 0; i < image_width; i++) {
                 auto r = double(i) / (image_width-1);
                 auto g = double(j) / (image_height-1);
-                auto b = 0;
+                auto b = 0.0;
 
                 int ir = int(255.999 * r);
                 int ig = int(255.999 * g);