Full-screen games on a Retina Macbook Pro sometimes start strangely
Thanks @eternus for answering, but my issue was that for some reason, my computer doesn't always switch to the discrete card once I open a game. Restarting usually fixes the problem, but a more thorough way to solve it is to get this app called gfxCardStatus, which allows you to specify which card you'd like to use. You choose the "Discrete only" option before running a game, and it should work fine. Here's what it looks like running in your task bar: