Fix simple.jl.

- Added two C_NULL arguments to GLFW.CreateWindow; there's only one method defined and it expects monitor::Ptr{None} and share::Ptr{None} arguments
- Check whether the return value of GLFW.WindowShouldClose is zero by == comparison rather than the boolean \!GLFW.WindowShouldClose(window), which throws an error because WindowShouldClose returns an int and ! is defined for Bools and BitArrays.