fix 3061 (white rect issue on glfw)

made 	glfwWindowHint(GLFW_VISIBLE,GL_FALSE);
global

fixes issue: https://github.com/openframeworks/openFrameworks/issues/3061

tested on PC&mac, please Test on LINUX


Note: 

this will "break" ofxSecondWindow addon because we now have to call show window after creating a new window, glfwWindowHint are global settings , we can fix it by reverting it from here but I think is better to fix the addon.

I will later make a seperate PR there as well.
we don't need this here anymore.