X11: Fix window creation hangs when another application is fullscreen (#1248)

* X11: Fix window creation hangs when another application is fullscreen

Previously, the X11 backend would block until a `VisibilityNotify` event
is received when creating a Window that is visible or when calling
`set_visible(true)` on a Window that is not currently visible. This
could cause winit to hang in situations where the WM does not quickly
send this event to the application, such as another window being
fullscreen at the time.

This behavior existed to prevent an X protocol error caused by setting
fullscreen state on an invisible window. This fix instead stores desired
fullscreen state when `set_fullscreen` is called (iff the window is not
visible or not yet visible) and issues X commands to set fullscreen
state when a `VisibilityNotify` event is received through the normal
processing of events in the event loop.

* Add window_debug example to facilitate testing

* Add a CHANGELOG entry

* Call `XUnmapWindow` if `VisibilityNotify` is received on an invisible window

src/platform_impl/linux/x11/event_processor.rs

@@ -497,6 +497,13 @@ impl<T: 'static> EventProcessor<T> {
                 });
             }
 
+            ffi::VisibilityNotify => {
+                let xev: &ffi::XVisibilityEvent = xev.as_ref();
+                let xwindow = xev.window;
+
+                self.with_window(xwindow, |window| window.visibility_notify());
+            }
+
             ffi::Expose => {
                 let xev: &ffi::XExposeEvent = xev.as_ref();