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
Implement more intelligent pixel blocking for SensorPathTracingIntegrator

Feature-End: multi-core
Add support for specifying a crop window for PerspectiveCamera
Add SampleConfig parameter to sampler.GenerateSamples() and use it

Feature-Start: sample-config
Make Sensor emit a SampleCnnfig for the Sample passed into SampleRay()
Generate samples for SensorPathTracer
Generate samples for SensorPathTracer's russian roulette

Also bound the numbers of samples generated for SensorPathTracer.
Pre-allocate storage for samples

Feature-End: sample-config
Implement Rect shape

Feature-Start: rect
Use a rect for the back wall in the Cornell box example
Use a rect for the front wall in the Cornell box example
Use a rect for the top light in the Cornell box example
Use a rect for the floor in the Cornell box example
Use a rect for the left wall in the Cornell box example
Use a rect for the right wall in the Cornell box example

Feature-End: rect