[crossfire] Weather system

[Nicolas Weeger (Laposte)]

Hello.

Following different bugfixes in the code, it seems to me the weather code is 
now operational, even at level 5.

The tests I made seem to be correct. Weather correctly changes tiles, puts 
snow/rain, in short behaves like it should from the code, and like one would 
expect.

Rednexela made some tests too, maybe he found something i missed :)


The fun is that, if weather code is fixed, it would mean it wasn't bugged (ok, 
there was one small logic issue that we fixed) in the first place, other 
parts of the code were responsible for the missing tiles and trees appearing 
on the sea :)


Nicolas