Handle scale factor change on web-sys backend (#1690)

* Change web backend `event_handler` to without  'static lifetime

* Refactor web runner and fix ControlFlow::Exit not being sticky

* Impl. scaling change event for web-sys backend

* Improve `dpi` docs regarding the web backend

* Add changes to changelog

* Update features.md

src/event_loop.rs

@@ -72,8 +72,11 @@ impl<T> fmt::Debug for EventLoopWindowTarget<T> {
 #[derive(Copy, Clone, Debug, PartialEq, Eq)]
 pub enum ControlFlow {
     /// When the current loop iteration finishes, immediately begin a new iteration regardless of
-    /// whether or not new events are available to process. For web, events are sent when
-    /// `requestAnimationFrame` fires.
+    /// whether or not new events are available to process.
+    ///
+    /// For web, events are queued and usually sent when `requestAnimationFrame` fires but sometimes
+    /// the events in the queue may be sent before the next `requestAnimationFrame` callback, for
+    /// example when the scaling of the page has changed.
     Poll,
     /// When the current loop iteration finishes, suspend the thread until another event arrives.
     Wait,