Added volumePhotonCubicIllum() to evaluate cubic illuminance from lightflow pmap. Two variants are currently being tested: (1) a single spherical lookup with a posteriori accumulation of photon flux to each axis illuminance based on photon's incident direction and normal, and (2) hemispherical lookups per axis with on-the-fly filtering based on axis normal; the latter is slower but more accurate, as it adjusts the to local density of light incident on each axis. Tests also show that a cosine term improves results with asymmetric illuminance, although this is not mathematically indicated.