Book 3.10.2: Sampling Directions towards a Hittable - Updates to Hittable List Missing

[hollasch]

Discussed in #1256

^{Originally posted by davidfrogley September 7, 2023}
Code listings in chapter 10.2 do not include changes to the hittable_list class, which should include an override of the pdf_value and generate methods from the hittable base class. Without these, the rendered image uses the default value for both of these (0 for pdf_value, and vec3(1,0.0) for generate). This results in a completely black image, except for the light on the ceiling.

[hollasch]

Now covered in #1318.