Pull out Russian roulette functionality into its own class

Feature-Start: particle-tracer
Add scaffolding for ParticleTracer and ParticleTracingRenderer

Also split hollow sphere scenes into two versions: one that uses
a path tracer and another that uses a particle tracer.
Add GetSampleConfig() and SampleRay() methods to Light

Implement those methods for DiffuseAreaLight.
Add parameter for transport type to Material
Add SampleConfig.CombineWith() method

This will be used to generate a SampleConfig that can be used for any
light.
Add more scaffolding to ParticleTracingRenderer and ParticleTracer

Determine sample count from sensors, and also handle the emitInterval
option.
Add ComputePixelPositionAndWe() method to Sensor
Pull out common radiometric functionality into Radiometer class
Add spectrumEstimatorPair class and make Radiometer use it

This is in preparation for supporting sampling starting from light
sources.
Add support for light-sampled contributions to Image class

This is in preparation for supporting samples coming from light
sources.
Add support for writing separate images for sensor and light pixels
Add methods to Sensor for accumulating light samples and recording them

Also add method for accumulating debug info.

Rename and comment existing Sensor methods to mention that they are
for samples coming from the sensor.
Add sensor list to Intersection type
Implement accumulation of alpha onto sensors
Make sampling method for DiffuseAreaLight configurable

Also add support for cosine sampling in DiffuseAreaLight.
Avoid modifying Sensors directly in ParticleTracer.SampleLightPath()
Implement accumulation and recording of debug info for ParticleTracer

Record the product of We and alpha for each edge count for level 1, and
We and alpha separately for level 2.
Add option to limit the edge count used in debugging in ParticleTracer
Implementing blocking for ParticleTracingRenderer to bound memory usage
Use multiple goroutines to process samples in ParticleTracingRenderer

Also switch back to counting samples for progress output.

Feature-End: particle-tracer