Use normal texture shaders in terrain ShaderPass.

Adapt passed parameters so a version of the terrain shaders using a
normal texture instead of per-vertex normals can be used.
Enable normal texture generation for Ogre Terrain.

Tell the Ogre Terrain Component to generate a normal texture for our
custom material. Also link the generated normal texture to an alias
defined in our material so it is placed in the correct texture unit.
Clean up old comments in ShaderPass.