Add an example that calls request_redraw() from a thread (#1467)

reproduces https://github.com/rust-windowing/winit/issues/1466

examples/request_redraw_threaded.rs

@@ -0,0 +1,39 @@
+use std::{thread, time};
+
+use winit::{
+    event::{Event, WindowEvent},
+    event_loop::{ControlFlow, EventLoop},
+    window::WindowBuilder,
+};
+
+fn main() {
+    simple_logger::init().unwrap();
+    let event_loop = EventLoop::new();
+
+    let window = WindowBuilder::new()
+        .with_title("A fantastic window!")
+        .build(&event_loop)
+        .unwrap();
+
+    thread::spawn(move || loop {
+        thread::sleep(time::Duration::from_secs(1));
+        window.request_redraw();
+    });
+
+    event_loop.run(move |event, _, control_flow| {
+        println!("{:?}", event);
+
+        *control_flow = ControlFlow::Wait;
+
+        match event {
+            Event::WindowEvent { event, .. } => match event {
+                WindowEvent::CloseRequested => *control_flow = ControlFlow::Exit,
+                _ => (),
+            },
+            Event::RedrawRequested(_) => {
+                println!("\nredrawing!\n");
+            }
+            _ => (),
+        }
+    });
+}