Yesterday, we finally had this meeting, and I discovered that the one that really wanted to throw me out of the paper was not Gunther, but his girlfriend ! This changed everything, and he was just "helping" her by intentionally breaking my code so that it could not be used anymore, and rewriting the paper from the first version and not from the last that I did.
The thing is that even by removing all my contributions, he could not remove the fact that I worked on the paper (and basically did his job of first author) and that the girlfriend could not have coded what she coded if I did not explain her the basics of sampling probability densities and fix the path tracer which was bugged. So after a long fight, in the middle of which Gunther simply leaved 'cause he had better things to do with his time as usual, the girlfriend made an offer which I accepted. It was again trying to find a bug. Which turned out not to be a real bug (I think, hard to say, it's not my original code), but more an algorithmic problem. Anyway, I found it and provided her with a solution, so she accepted to put me on the paper. Of course, as they removed all my work, I won't be second place anymore, but after all, I don't care as long as I do not do any writing or coding anymore !
I'm happy because they wanted to screw me up, but did not completely manage. For once, Gunther and his machiavelic plans did not win ! I just hope it's really over, and, please, reviewers, accept this paper because I never want to have anything to do with Gunther or his girlfriend ever !