Explain the one-second relights in a comment

Should make it a bit quicker for the next person to figure out what is
going on there...
Fix FoW deinitialization in C4Landscape
Remove two unused declarations in C4Landscape

There are no implementations anymore for these.
Apply landscape re-lighting again in every frame

This was accidentally removed in commit
56bdeec5850d396522d7f4d47154eaf945e8c90d. It fixes the landscape being
only updated once every second when digging.
Add functionality to update the ambient map at runtime
Update ambient map on landscape changes