Include probability of initial part of the path #8
Description
Currently, the path tracer disregards the probability density function of the initial ray shot from the camera; the probability of shooting through a particular point of a pixel is therefore not accounted for, and is assumed to be 1. This leads to a bias (not overly noticeable in this example) in the Monte Carlo integration.
This should be fixed by also taking the PDF of the initially generated path segment as well, in the beginning of the path tracing implementation.