Implement headless rendering

examples/window.rs

@@ -12,6 +12,10 @@ mod support;
 #[cfg(target_os = "android")]
 android_start!(main)
 
+#[cfg(not(feature = "window"))]
+fn main() { println!("This example requires glutin to be compiled with the `window` feature"); }
+
+#[cfg(feature = "window")]
 fn main() {
     let window = glutin::Window::new().unwrap();