Add SamplePixelPositionAndWeFromPoint() method to Sensor

Write non-trivial implementations for IrradianceMeter, FluxMeter, and
PerspectiveCamera.
Add Light methods to sample/compute spatial and directional Le seperately

Also implement them for DiffuseAreaLight.
Implement ParticleTracer sensor sampling method

Use that method in all particle tracer scenes.

Feature-End: particle-tracer-sensor-sampling