Implement requestAnimationFrame for web (#1519)

* Use requestAnimationFrame for polling wasm

* Implement `requestAnimationFrame` for stdweb

Co-authored-by: Ryan G <ryanisaacg@users.noreply.github.com>

src/platform_impl/web/event_loop/state.rs

@@ -15,7 +15,7 @@ pub enum State {
         start: Instant,
     },
     Poll {
-        timeout: backend::Timeout,
+        request: backend::AnimationFrameRequest,
     },
     Exit,
 }