Adds normal/bump mapping support - normal map atlas support is missing though...

* Adds simplified tangent space calculation to the chunk shader
* Adds normal mapping support to the chunk shader
* Adds a setting to the rendering config which can be used to enable/disable normal mapping
* Adds an example stone normal map
* Fixes that the Lambertian lighting helper method could return values < 0
* Adds a shader helper method which can be used to generate normalized texture coordinates from an arbitrary atlas texture position
* Adds various cleanups in the shaders that I made while I was at it