Would it be better to create a callback function which contains a OpenGL 3.0+ Context including viewport, matrix, etc or setup OpenGL in a function called GL_StartUp and use that GL_StartUp Function in the mainloop and callback function to that Function.

I want my program to only show a OpenGL default scene when the user clicks on the New Game menu item in the menu bar rather then just have one setup when the program starts.

I'm using Ubuntu 64bit, GTK 3.0 and GTK OpenGL