Parameters
cbfun
Pointer to a callback function that will be called when the window client area needs to be
refreshed. The function should have the following C language prototype:
void GLFWCALL functionname( void );
Where functionname is the name of the callback function.
If cbfun is NULL, any previously selected callback function will be deselected.

Description
The function selects which function to be called upon a window refresh event, which occurs when any
part of the window client area has been damaged, and needs to be repainted (for instance, if a part of the
window that was previously occluded by another window has become visible).
A window has to be opened for this function to have any effect.

Notes
Window refresh events are recorded continuously, but only reported when glfwPollEvents,
glfwWaitEvents or glfwSwapBuffers is called.