Fixes #623.
Adds some minor cleanups in the light geometry pass fragment shader.
Adds support for the automated generation of height- and and normal map texture atlases and some minor fixes. All you have to do is create a new tile texture png with the suffixes "Height" or "Normal". If you like to specify a explicit name for the asset, you can use the "tileNormal" and "tileHeight" members of the Block Definition class. So a normal map for a stone block can be created by adding a "StoneNormal.png" texture to the assets directory.

* Adds support for sampling height and normal map texture atlases to the chunk fragment shader
* Removes parallax stone texture because it forced a texture atlas size of 4096*4096. Back to 256*256...
* Fixes a shader compile issue on the lower visual settings
Revision of the "books" mod.
* Made bookcases stackable
* when destroyed, bookcases drop their content - be careful with 99-er stacks ;)
* consistent naming (bookshelf -> bookcase)
* package structuring