I didn't make it!
I have successfully initialised the Clip Coordinate mode, but now my aspect ratio is wrong (e.g. a box is wider than higher on the screen). I was able to get rid of that by changing the perspective transformation so that different Values were used for X and Y coordinates... The problem in the moment is that the texture coordinates seem to be garbled up when one rotates the faces, so there has to be a problem with the texture coords or q. I have corrected u and v to a range of 0..1, and q is set to 1. I would be most grateful if anyone knew a solution to this problem, or could post a short code fragment showing how he (or she, that is ;) solved that.