Terrain normals

[fjansson]

I've found that normals of the terrain entity vary too little, i.e. they point mostly upwards even if the terrain is sloping.

The current code:
models/procedural/terrain.py

  rl =  _height_values[x+1][z] - _height_values[x-1][z]
  fb =  _height_values[x][z+1] - _height_values[x][z-1]
  self.normals.append(Vec3(rl, 1, fb).normalized())

It seems the two derivatives rl and fb should be scaled with the mesh size, as for example here:
https://stackoverflow.com/questions/6656358/calculating-normals-in-a-triangle-mesh/21660173#21660173

I tried the following and it works much better (still not sure if there's a factor 2 missing):

  rl =  (_height_values[x+1][z] - _height_values[x-1][z]) * (w-1)
  fb =  (_height_values[x][z+1] - _height_values[x][z-1]) * (h-1)
  self.normals.append(Vec3(rl, 1, fb).normalized())