Web: Fix incorrect scale when moving to screen with new DPI (#5631)

* Closes https://github.com/emilk/egui/issues/5246

Tested on
* [x] Chromium
* [x] Firefox
* [x] Safari

On Chromium and Firefox we get one annoying frame with the wrong size,
which can mess up the layout of egui apps, but this PR is still a huge
improvement, and I don't want to spend more time on this right now.

crates/eframe/Cargo.toml

@@ -211,6 +211,7 @@ percent-encoding = "2.1"
 wasm-bindgen.workspace = true
 wasm-bindgen-futures.workspace = true
 web-sys = { workspace = true, features = [
+  "AddEventListenerOptions",
   "BinaryType",
   "Blob",
   "BlobPropertyBag",