class lambertian should have constructors for color or texture

[hollasch]

In the second book, the lambertian material is converted from taking a color in the constructor to a texture, changing this:

lambertian(const color& a) : albedo(a) {}

to this

lambertian(shared_ptr<texture> a) : albedo(a) {}

In the process, the reader is told to go back over their code and update all lambertian values to now use the new solid_color texture where they used to have color, so

...make_shared<lambertian>(color(0.5, 0.5, 0.5))

should get updated to

...make_shared<lambertian>(make_shared<solid_color>(0.5, 0.5, 0.5))

Seems like it would be easier to have two lambertian constructors, like so:

lambertian(const color& a) : albedo(make_shared<solid_color>(a)) {}
lambertian(shared_ptr<texture> a) : albedo(a) {}

The result is that the lambertian can take either color or texture, and the reader doesn't have to go back to update everything.