How does Xonotic stretch to all my screens on Linux?

Didn't think it was possible on Linux, but I started up Xonotic, and to my surprise, it stretched out on all 3 of my screens with an in-game resolution of 4800x900 (was expecting it to just stick to my one 1600x900 screen or mirror across all 3).

I'm wondering if there's some sort of way to replicate what Xonotic is doing in order to use it on other games?

Check out darkplaces/vid_*.c and see how it's done.