More options for Shadow Bias/Min Ray Dist and fix for Non-IBL Background caustics

Hello,

Main changes in this Core beta4 version respect to beta3:

• New advanced parameters for fine control of Shadow Bias/Min Ray Dist if the automatic calculation is not good enough.
• Fix for caustics noise coming from Non-IBL background in Photon Mapping. Now the background will only be used for Caustics if IBL is enabled and (if there is a caustics parameter available in the background settings) if caustics is enabled in the background.