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()
 };
```

crates/eframe/src/icon_data.rs

@@ -0,0 +1,62 @@
+//! Helpers for loading [`egui::IconData`].
+
+use egui::IconData;
+
+/// Helpers for working with [`IconData`].
+pub trait IconDataExt {
+    /// Convert into [`image::RgbaImage`]
+    ///
+    /// # Errors
+    /// If `width*height != 4 * rgba.len()`, or if the image is too big.
+    fn to_image(&self) -> Result<image::RgbaImage, String>;
+
+    /// Encode as PNG.
+    ///
+    /// # Errors
+    /// The image is invalid, or the PNG encoder failed.
+    fn to_png_bytes(&self) -> Result<Vec<u8>, String>;
+}
+
+/// Load the contents of .png file.
+///
+/// # Errors
+/// If this is not a valid png.
+pub fn from_png_bytes(png_bytes: &[u8]) -> Result<IconData, image::ImageError> {
+    crate::profile_function!();
+    let image = image::load_from_memory(png_bytes)?;
+    Ok(from_image(image))
+}
+
+fn from_image(image: image::DynamicImage) -> IconData {
+    let image = image.into_rgba8();
+    IconData {
+        width: image.width(),
+        height: image.height(),
+        rgba: image.into_raw(),
+    }
+}
+
+impl IconDataExt for IconData {
+    fn to_image(&self) -> Result<image::RgbaImage, String> {
+        crate::profile_function!();
+        let Self {
+            rgba,
+            width,
+            height,
+        } = self.clone();
+        image::RgbaImage::from_raw(width, height, rgba).ok_or_else(|| "Invalid IconData".to_owned())
+    }
+
+    fn to_png_bytes(&self) -> Result<Vec<u8>, String> {
+        crate::profile_function!();
+        let image = self.to_image()?;
+        let mut png_bytes: Vec<u8> = Vec::new();
+        image
+            .write_to(
+                &mut std::io::Cursor::new(&mut png_bytes),
+                image::ImageOutputFormat::Png,
+            )
+            .map_err(|err| err.to_string())?;
+        Ok(png_bytes)
+    }
+}