Fill the windows in the examples with a solid color

Fixes #776.

examples/cursor_grab.rs

@@ -8,6 +8,9 @@ use winit::{
     window::{CursorGrabMode, WindowBuilder},
 };
 
+#[path = "util/fill.rs"]
+mod fill;
+
 fn main() {
     SimpleLogger::new().init().unwrap();
     let event_loop = EventLoop::new();
@@ -67,6 +70,7 @@ fn main() {
                 },
                 _ => (),
             },
+            Event::RedrawRequested(_) => fill::fill_window(&window),
             _ => (),
         }
     });