Refactor the terrain system

The current terrain system works, but it's hard to make it support different materials for shader capable and non-shader capable cards. We also want to use normal mapped shaders for the terrain, something which requires a new way of handling the terrain data.
The current TerrainPage class needs to be refactored into these classes: "TerrainPageGeometry" being responsible for the geometry of the page, "TerrainPageSurface" being responsible for the texture of the page, "TerrainPageSurfaceLayer" being responsible for one mercator shader, "TerrainPageSurfaceCompiler" being responsible for creating an Ogre material from the aforementioned classes.
The "TerrainPageSurfaceCompiler" will then use many different "TerrainPageSurfaceCompilerBackend" classes which will create the actual materials depending on what the current graphics card support.