Add scaffolding for processing light image contributions

Feature-Start: bdpt-light-image
implement light contributions
rem check
relax checks
remove las restriction
one more
hack up
undo check
fix path count
add isspecular
use isspecular
implement sensor ComputePixelPosition
fix light contrib
fix "irradiance"
fix pinhole
fix?
one more fix
fix contribution type
fix debug
remove accum