Move LightMaterial to light.go
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()
add sensormaterial
implement vertexqs
Implement Sensor.ComputeWeSpatialPdf()
Return surface information from Sensor.SamplePixelPositionAndWeFromPoint()
implement sensor
implement directional
Make all scenes using TwoWayPathTracer use power weighing method