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.
Add direct sensor path type to ParticleTracer

Use it for most scenes.

Feature-End: particle-tracer-direct-sensor-sampling