Buffer refactoring

This pull request represents the completion of the second deliverable of my Google Summer of Code project.
I refactored the zbuffer code and made it into a proper c++ struct, I also moved all the functions inside the struct.
The zfill functions were refactored to support 32 bpp pixel textures only and the code was simplified to use only texture.getValueAt.
If you have any questions or doubts please ask.