demo/glfw_opengl3 Remove global state

[riri]

Port vurtun PR, fixes #107

@maddouri statement:

Thanks for your interest in the PR. IMHO, not having a global state ought to be a requirement for all code samples.
Unfortunately, I doubt that I would have enough resources to dedicate to porting vurtun/nuklear#760 to this repo.

To anyone interested: please feel free to adapt my PR's code to this new repo :)

[Hejsil]

Looks good!

[riri]

I just realized that putting the implementation in main source is needed to make the backend available, because the struct nk_glfw is not defined in the "header" part.

Wouldn't it be better to move the definition of struct nk_glfw before the NK_GLFW_GL3_IMPLEMENTATION part?