add parent window setting when creating windows with glutin

needed for embedding api usage

components/servo/main.rs

@@ -53,7 +53,7 @@ fn main() {
         let window = if opts::get().headless {
             None
         } else {
-            Some(app::create_window())
+            Some(app::create_window(std::ptr::null_mut()))
         };
 
         // Our wrapper around `Browser` that also implements some