WIP: GLSL function composition

Basically adds two files: the new Function implementation and a file with examples and tests. High level users should mostly checkout the docstring of the Function class, and the examples in test_function.py.

Everything works (or should, I hope to add a test or two). I think the only remaining issue is that I am not sure yet how variables should work out. 

No need to wait for me to move this forward. If I don't like what you've done with it, I'll nag and we can re-discuss afterwards ;)

This example was actually made to testdrive the Function mechanism. I actually feel we're not done yet, because we can possibly simplify some things. I am playing with a form of light-weight components. So you can consider these examples aimed at devs for now. Later we need to turn them in user-oriented ones :)