Fixed buggy handling of contribMode in distribPhotonContrib() and avoided divide by zero srcFlux normalisation if some light sources didn't emit any photons. Most importantly, detected MAJOR error in shared photon counter updates in distribPhotons() and distribPhotonContrib() as they number of emitted photons reported to the parent process is a multiple of PMAP_CNTUPDATE, meaning the photon flux was staggered!!! This bug has been corrected in distribPhotons() with a simple scalar, but is more complicated in distribPhotonContrib() as there are separate emission counters per light source, requiring an array. THE LATTER IS TODO!