Now App::update will be responsabile for what viewport will be drawned and its content

A big refactoring was needed

examples/screenshot/src/main.rs

@@ -1,6 +1,6 @@
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 
-use eframe::egui::{self, ColorImage};
+use eframe::egui::{self, ColorImage, ViewportRender};
 
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
@@ -24,7 +24,16 @@ struct MyApp {
 }
 
 impl eframe::App for MyApp {
-    fn update(&mut self, ctx: &egui::Context, frame: &mut eframe::Frame) {
+    fn update(
+        &mut self,
+        ctx: &egui::Context,
+        frame: &mut eframe::Frame,
+        render: Option<&ViewportRender>,
+    ) {
+        if let Some(render) = render {
+            render(ctx, frame.viewport_id(), frame.parent_viewport_id());
+            return;
+        }
         egui::CentralPanel::default().show(ctx, |ui| {
             if let Some(screenshot) = self.screenshot.take() {
                 self.texture = Some(ui.ctx().load_texture(