Continue from PathTracer1, add more complicated materials, environment lights, and depth of field to ray tracer.
With sample rate of 256, 4 samples per light and max depth of 4. With sample rate of 256, 4 samples per light and max depth of 7.
Importance sampling is introduced to increase the density of sample points in the regions that we are interested in.
Change aperture and focal distance to control the depth of field.