Function RB_DetermineLightScale

================= RB_DetermineLightScale Sets: backEnd.lightScale backEnd.overBright Find out how much we are going to need to overscale the lighting, so we can down modulate the pre-lighting passes. We only look at light calculations, but an argument could be made that we should also look at surface evaluations, which would let surfaces overbright past 1.0 ================= */