Fill the windows in the examples with a solid color

Fixes #776.

examples/window_drag_resize.rs

@@ -10,6 +10,9 @@ use winit::{
 
 const BORDER: f64 = 8.0;
 
+#[path = "util/fill.rs"]
+mod fill;
+
 fn main() {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
@@ -65,6 +68,9 @@ fn main() {
             }
             _ => (),
         },
+        Event::RedrawRequested(_) => {
+            fill::fill_window(&window);
+        }
         _ => (),
     });
 }