Add a Fog: trait to enable fog of war in ShroudRenderer

This makes fog of war conditional for nostalgic mappers and classic mods. It was the reason https://github.com/Iran/ClassicRA forked us and has been requested multiple times by die-hard fans of the original:

- http://www.sleipnirstuff.com/forum/viewtopic.php?f=83&t=16023
- http://cnc-comm.com/community/index.php?topic=1619.msg12285#msg12285
- http://forum.dune2k.com/topic/24184-opend2k-cross-platform-reimplementation-of-dune-2000/?p=377815