Refraction shader

Hi there, I made a pull request on github (here) that is implementing refraction and fixing a bad issue with reflection : In case you tried to use ssr with large objects, you probably noticed that they just don’t reflect entirely, due to a slight error in the raycasting steps computation.
This is fixes and you also get refraction.
In case you want the fork before it’s merge, checkout to ss_refraction.
Bye.