demo/glfw_opengl3 Remove global state

[maddouri]

Hi,

Its great that Nuklear doesn't depend on globals. However, some of the demos do. This could be a problem if we want to create multiple windows with their own, separate states/contexts. (e.g. #451)

This PR is an attempt to get rid of the global state for the glfw_opengl3 demo.

EDIT: I forgot to mention that:

1. I have also fixed what appears to be a typo:
   • nk_gflw3_scroll_callback() -> nk_glfw3_scroll_callback()
2. I could remove the global state from other GLFW demos if this PR is accepted