Most mobs disappear from map at night

[Plastikmensch]

Currently block light is the determining factor of whether a mob is considered hidden or not.
The issue is that block light changes based on time of day.
A block without a nearby light source therefore has a block light value of 15 (as sky light is considered in the calculation) during the day and 0 during night (while sky light still stays the same, but isn't considered in the calculation anymore).
Possible fix: Determine whether a mob is in a shadow based by a combination of sky light and block light.