Make terrain editing work again with Ogre Terrain

- Implement reloadPage() in OgreTerrainAdapter.
- Change signature to just take a Domain::TerrainIndex since the
  conversion should be the responsibility of the terrain
  implementation.
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.
Keep texture aliases on terrain material reload

The terrain materials are sometimes reloaded for various reasons.
Texture aliases set in order to link the terrain normal texture were lost
when doing this. With this patch these aliases are saved and re-applied
when reinitializing the materials, which avoids having to reload the
entire Ogre terrain.
Avoid unnecessary terrain material recompilation
Add some comments and log outputs.
Remove unused includes from TerrainPageGeometry
Disable shadow casting on terrain, tweak params
Disable debug display on terrain page manager