Use egui::ViewportBuilder in eframe::NativeOptions (#3572)

* Part of https://github.com/emilk/egui/issues/3556

This PR replaces a bunch of options in `eframe::NativeOptions` with
`egui::ViewportBuilder`. For instance:

``` diff
 let options = eframe::NativeOptions {
-    initial_window_size: Some(egui::vec2(320.0, 240.0)),
-    drag_and_drop_support: true,
+    viewport: egui::ViewportBuilder::default()
+        .with_inner_size([320.0, 240.0])
+        .with_drag_and_drop(true),
     centered: true,
     ..Default::default()
 };
```

examples/test_viewports/src/main.rs

@@ -12,10 +12,11 @@ fn main() {
     let _ = eframe::run_native(
         "Viewports",
         eframe::NativeOptions {
+            viewport: egui::ViewportBuilder::default().with_inner_size([450.0, 400.0]),
+
             #[cfg(feature = "wgpu")]
             renderer: eframe::Renderer::Wgpu,
 
-            initial_window_size: Some(egui::Vec2::new(450.0, 400.0)),
             ..Default::default()
         },
         Box::new(|_| Box::<App>::default()),