Add the ability to pass a prebuilt canvas (#1394)

This allows Winit to take control of existing canvas elements in the
DOM, which is useful for web applications with other content in the
page.

src/platform_impl/web/stdweb/mod.rs

@@ -72,3 +72,5 @@ pub fn is_fullscreen(canvas: &CanvasElement) -> bool {
         None => false,
     }
 }
+
+pub type RawCanvasType = CanvasElement;