Fill the windows in the examples with a solid color

Fixes #776.

examples/mouse_wheel.rs

@@ -7,6 +7,9 @@ use winit::{
     window::WindowBuilder,
 };
 
+#[path = "util/fill.rs"]
+mod fill;
+
 fn main() {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
@@ -56,6 +59,9 @@ In other words, the deltas indicate the direction in which to move the content (
                 },
                 _ => (),
             },
+            Event::RedrawRequested(_) => {
+                fill::fill_window(&window);
+            }
             _ => (),
         }
     });