Add Sensor.HasSpecular{Position,Direction}() methods

Implement those methods for all sensors.

Feature-Start: particle-tracer-combined-paths
Allow for combination of path types in ParticleTracer

Make all particle tracing test scenes use both emitted importance paths
and direct sensor paths.
Emit more debug info in ParticleTracer

Move existing debug info up one level, and add a few new pieces of
debug info for levels 1 and 2.

Feature-End: particle-tracer-combined-paths