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

A big refactoring was needed

crates/egui_demo_app/src/apps/custom3d_glow.rs

@@ -1,7 +1,7 @@
 use std::sync::Arc;
 
 use eframe::egui_glow;
-use egui::mutex::Mutex;
+use egui::{mutex::Mutex, ViewportRender};
 use egui_glow::glow;
 
 pub struct Custom3d {
@@ -21,7 +21,16 @@ impl Custom3d {
 }
 
 impl eframe::App for Custom3d {
-    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| {
             egui::ScrollArea::both()
                 .auto_shrink([false; 2])