Improved sampling of flat photon ports / light sources; added specific code to handle disk and triangular sources in flatPhotonOrigin(), as well as (inefficient) rejection sampling for arbitrary polygons. The generalised routine photonOrigin() now returns an acceptance flag to signal photon rejection/acceptance. distribPhotons() now skips rejected photons, and does not increment the emission counter to compensate, thus increasing the photon flux. Unfortunately, this is not without side effects, as it affects the progress counter AND produces fewer photons than desired...