implement addvertexqs for pathtracer
Implement ShapeSet.ComputePdf()
Add Light.ComputeLeSpatialPdf() method and implement for DiffuseAreaLight
Return surface information from Light.SampleLeFromPoint()
implement light spatial qs
implement directional
Make Light.SampleRay() return a pdf value
keep track of pdflight
Make Light.SampleSurface() return a pdf value
keep track of pdf spatial/directional
Implement Sensor.ComputeWeDirectionalPdf()
implement vertexqs
Implement Sensor.ComputeWeSpatialPdf()
Return surface information from Sensor.SamplePixelPositionAndWeFromPoint()
implement sensor
implement directional
Make all scenes using TwoWayPathTracer use power weighing method