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()
 };
```

.vscode/settings.json

@@ -20,14 +20,16 @@
         "--workspace",
         "--message-format=json",
         "--all-targets",
+        "--all-features",
     ],
     "rust-analyzer.cargo.buildScripts.overrideCommand": [
         "cargo",
-        "check",
+        "cranky",
         "--quiet",
         "--target-dir=target_ra",
         "--workspace",
         "--message-format=json",
         "--all-targets",
+        "--all-features",
     ],
 }