Add screenshot support for eframe web (#5438)

This implements web support for taking screenshots in an eframe app (and
adds a nice demo).
It also updates the native screenshot implementation to work with the
wgpu gl backend.

The wgpu implementation is quite different than the native one because
we can't block to wait for the screenshot result, so instead I use a
channel to pass the result to a future frame asynchronously.

* Closes <https://github.com/emilk/egui/issues/5425>
* [x] I have followed the instructions in the PR template


https://github.com/user-attachments/assets/67cad40b-0384-431d-96a3-075cc3cb98fb

crates/eframe/src/web/web_painter.rs

@@ -1,3 +1,4 @@
+use egui::{Event, UserData};
 use wasm_bindgen::JsValue;
 
 /// Renderer for a browser canvas.
@@ -16,14 +17,19 @@ pub(crate) trait WebPainter {
     fn max_texture_side(&self) -> usize;
 
     /// Update all internal textures and paint gui.
+    /// When `capture` isn't empty, the rendered screen should be captured.
+    /// Once the screenshot is ready, the screenshot should be returned via [`Self::handle_screenshots`].
     fn paint_and_update_textures(
         &mut self,
         clear_color: [f32; 4],
         clipped_primitives: &[egui::ClippedPrimitive],
         pixels_per_point: f32,
         textures_delta: &egui::TexturesDelta,
+        capture: Vec<UserData>,
     ) -> Result<(), JsValue>;
 
+    fn handle_screenshots(&mut self, events: &mut Vec<Event>);
+
     /// Destroy all resources.
     fn destroy(&mut self);
 }