Compute light colors depending on time of day / sun vector

This PR makes SunPosition.js compute and apply different light and shadow colors depending on the time of day. A transition exponent is available to tweak the steepness of the day/night transition.

I left a commented console.log in there to allow inspection of the sun/up vector angle in case the sun vector is updated, maybe to support DST (night tends to fall around 17~18h here).