Add scaffolding for PathTracer.SampleWeightedLi()
Add PrimitiveList
Add GeometricPrimitive and more scaffolding
Add Sphere and more scaffolding
Flesh out Ray
Add Diffuse material
Use hollow sphere test scene

Feature-End: initial-scene
Implement cosine-weighted hemisphere sampling

Feature: cosine-weighting
Build the renderer from a config map

Feature-Start: read-config
Build the scene from a config map

Also add comments about hollow sphere scene.
Read config from file
Process any number of config files on the command line

Feature-End: read-config
Communicate sensor extent to renderer

Feature-Start: multiple-sensors
Make sensors part of the scene

Add GetSensors() function to Primitive.

Add PointPrimitive.

Add "black" Spectrum type.
Add hollow sphere with center light example

Feature-End: multiple-sensors
Add Cornell box example

Feature-Start: perspective-camera
Implement PerspectiveCamera sensor and use it in Cornell box example

Feature-End: perspective-camera
Render using multiple goroutines

Feature-Start: multi-core
Add flag to control number of render jobs
Add flag to write a CPU profile