message: Make window used for screenshot visible
  For some reason, this is required for
  the screenshot example to work,
  otherwise it would just create an empty
  image.
  Fixes
diff: @@ -9,7 +9,7 @@ use glium::index::PrimitiveType;
  fn main() {
  // building the display, ie. the main object
  let event_loop = glutin::event_loop::EventLoop::new();
  - let wb = glutin::window::WindowBuilder::new().with_visible(false);
  + let wb = glutin::window::WindowBuilder::new().with_visible(true);
  let cb = glutin::ContextBuilder::new();
  let display = glium::Display::new(wb, cb, &event_loop).unwrap();
  