Added locking of shared memmapped photon counter file in distribPhotons() and distribPhotonContrib() after more (erroneous) counter overflows due to race conditions. Also decorrelated photon counter update interval photonCntUpdate for each subprocess. Revised setting of photon's aux.contribSrc field for precomputed contrib photons in newPhoton() and its call in preComputeContrib(). This currently set, but its value is ignored since precomputed contrib pmaps are separated per modifier anyway.