Add access to the canvas in the Window

src/platform_impl/stdweb/window.rs

@@ -1,8 +1,9 @@
 use dpi::{LogicalPosition, LogicalSize, PhysicalPosition, PhysicalSize};
 use event::{Event, WindowEvent};
 use icon::Icon;
+use platform::stdweb::WindowExtStdweb;
 use monitor::{MonitorHandle as RootMH};
-use window::{CreationError, MouseCursor, WindowAttributes, WindowId as RootWI};
+use window::{CreationError, MouseCursor, Window as RootWindow, WindowAttributes, WindowId as RootWI};
 use super::{EventLoopWindowTarget, register};
 use std::collections::VecDeque;
 use std::collections::vec_deque::IntoIter as VecDequeIter;
@@ -317,3 +318,9 @@ impl Window {
         unsafe { WindowId::dummy() }
     }
 }
+
+impl WindowExtStdweb for RootWindow {
+    fn canvas(&self) -> CanvasElement {
+        self.window.canvas.clone()
+    }
+}