Added OpenGL(R) style comparison function

 - different comparison mode IDs
 - function to get comparison function implementation
 - comparison function used for depth test

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Added configurable backface culling

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>
Updated README

Signed-off-by: David Oberhollenzer <david.oberhollenzer@tele2.at>