Fill the windows in the examples with a solid color

Fixes #776.

examples/child_window.rs

@@ -1,3 +1,7 @@
+#[cfg(any(x11_platform, macos_platform, windows_platform))]
+#[path = "util/fill.rs"]
+mod fill;
+
 #[cfg(any(x11_platform, macos_platform, windows_platform))]
 fn main() {
     use std::collections::HashMap;
@@ -70,6 +74,10 @@ fn main() {
                 }
                 _ => (),
             }
+        } else if let Event::RedrawRequested(wid) = event {
+            if let Some(window) = windows.get(&wid) {
+                fill::fill_window(window);
+            }
         }
     })
 }