gladLoadGL() fails on Windows 8.1 x64
I'm compiling with glad.c and glad/glad.h from the "c" branch on github.

When I call gladLoadGL(), it fails, because glGetString() on GL_VERSION returns NULL. Before that fails, it has successfully loaded opengl32.dll and gotten the glGetString function pointer.

The system is Windows 8.1 x64, NVIDIA GTX 870M graphics, driver 340.52. This is an Optimus machine, so it also has an Intel HD 4600 for the actual display framebuffer.
