Implement missing GLX Offscreen rendering functions

I found out what the issue was! The CreateOffScreen functions were unimplemented for GLX, and were left as a {return true;} from the GLInterface.h file (really? return success if it's inimplemented? whyyyyy). With this, I... well, get the same crash that I get on the master branch, which is a segfault (presumably unrelated to VR!)

These GLX definitions are rough, and were simply copies from the typical definitions applied to seperate offscreen contexts, with some modifications whereever it seemed neccesary. I need to test them more, but I can't really until there's an upstream merge that fixes whatever's causing that segfault, and ofcourse my DK2 arrives.

So yeah! EGL, and a handful of other definitions need to be implemented. But I think the GLInterface.h file should at the very least have a prototype definition of {return false;}, since if that's the only function called, *it definately didn't succeed*.