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/confirm_exit/src/main.rs

@@ -5,7 +5,7 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/custom_3d_glow/src/main.rs

@@ -9,7 +9,7 @@ use std::sync::Arc;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(350.0, 380.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([350.0, 380.0]),
         multisampling: 4,
         renderer: eframe::Renderer::Glow,
         ..Default::default()

examples/custom_font/src/main.rs

@@ -5,7 +5,7 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/custom_window_frame/src/main.rs

@@ -7,12 +7,12 @@ use eframe::egui::{self, ViewportCommand};
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        // Hide the OS-specific "chrome" around the window:
-        decorated: false,
-        // To have rounded corners we need transparency:
-        transparent: true,
-        min_window_size: Some(egui::vec2(400.0, 100.0)),
-        initial_window_size: Some(egui::vec2(400.0, 240.0)),
+        viewport: egui::ViewportBuilder::default()
+            .with_inner_size([400.0, 100.0])
+            .with_min_inner_size([400.0, 100.0])
+            .with_decorations(false) // Hide the OS-specific "chrome" around the window
+            .with_transparent(true), // To have rounded corners we need transparency
+
         ..Default::default()
     };
     eframe::run_native(

examples/file_dialog/src/main.rs

@@ -5,8 +5,9 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        drag_and_drop_support: true,
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default()
+            .with_inner_size([320.0, 240.0])
+            .with_drag_and_drop(true),
         ..Default::default()
     };
     eframe::run_native(

examples/hello_world/src/main.rs

@@ -5,7 +5,7 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/hello_world_par/src/main.rs

@@ -10,7 +10,7 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(1024.0, 768.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([1024.0, 768.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/hello_world_simple/src/main.rs

@@ -6,7 +6,7 @@ fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
 
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
         ..Default::default()
     };
 

examples/images/src/main.rs

@@ -5,7 +5,7 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(600.0, 800.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([600.0, 800.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/multiple_viewports/src/main.rs

@@ -10,7 +10,7 @@ use eframe::egui;
 fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/save_plot/src/main.rs

@@ -7,7 +7,7 @@ fn main() -> Result<(), eframe::Error> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
 
     let options = eframe::NativeOptions {
-        initial_window_size: Some(egui::vec2(350.0, 200.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([350.0, 200.0]),
         ..Default::default()
     };
     eframe::run_native(

examples/serial_windows/src/main.rs

@@ -11,7 +11,7 @@ fn main() -> Result<(), eframe::Error> {
 
     let options = eframe::NativeOptions {
         run_and_return: true,
-        initial_window_size: Some(egui::vec2(320.0, 240.0)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([320.0, 240.0]),
         ..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()),

examples/user_attention/src/main.rs

@@ -6,7 +6,7 @@ use std::time::{Duration, SystemTime};
 fn main() -> eframe::Result<()> {
     env_logger::init(); // Log to stderr (if you run with `RUST_LOG=debug`).
     let native_options = NativeOptions {
-        initial_window_size: Some(egui::vec2(400., 200.)),
+        viewport: egui::ViewportBuilder::default().with_inner_size([400., 200.]),
         ..Default::default()
     };
     eframe::run_native(