Rewritten the way normals are calculated for the Terrain.cs
This is slower than the code from before but it produces near-correct results. There is the option to turn on supersmooth normals, but there are even slower and give only marginally better results.
git-svn-id: http://svn.uktrainsim.com/svn/openrails/trunk@326 a9f3cfff-6c41-7446-b0ca-052bd761e6eb