It seems rather ridiculous to ask graphics-programming questions of someone interviewing for a server side position. I don't think any kind of preparation would help you with that kind of questions.
I asked the recruiter I had been working with about it, and her response was along the lines of: 'We only hire engineers who are capable of tackling any problem faced by the company.' ...
So in other words they want that mystical 10x engineer
There is one kind of preparation: having graphics programming as a hobby, or failing that, an interest in everything under the sun and nonstop studying.