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

A big refactoring was needed

examples/custom_window_frame/src/main.rs

@@ -2,7 +2,7 @@
 
 #![cfg_attr(not(debug_assertions), windows_subsystem = "windows")] // hide console window on Windows in release
 
-use eframe::egui;
+use eframe::egui::{self, ViewportRender};
 
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
@@ -30,7 +30,16 @@ impl eframe::App for MyApp {
         egui::Rgba::TRANSPARENT.to_array() // Make sure we don't paint anything behind the rounded corners
     }
 
-    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;
+        }
         custom_window_frame(ctx, frame, "egui with custom frame", |ui| {
             ui.label("This is just the contents of the window.");
             ui.horizontal(|ui| {