Fill the windows in the examples with a solid color

Fixes #776.

examples/multiwindow.rs

@@ -10,6 +10,9 @@ use winit::{
     window::Window,
 };
 
+#[path = "util/fill.rs"]
+mod fill;
+
 fn main() {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
@@ -56,6 +59,11 @@ fn main() {
                     _ => (),
                 }
             }
+            Event::RedrawRequested(window_id) => {
+                if let Some(window) = windows.get(&window_id) {
+                    fill::fill_window(window);
+                }
+            }
             _ => (),
         }
     })