Big changes:
Switch to PathIntegrator for default Integrator
Switch to HaltonSampler for default Sampler
Switch to 16spp for default number of pixel samples.
Fix uninitialized memory bug in InitProfiler()

Smaller changes:
Add profiler category for camera ray generation.
Many naming cleanups, as usual.
Return exit pupil bounds from RealisticCamera::SampleExitPupil() to make
  ray weight computation more straightforward.
Use unique_ptr for Mapping[23]D in Texture implementations