Store web_sys::File inside of DroppedFile (#8354)

* Closes #4654
* Related #4667
* [x] I have followed the instructions in the PR template

This PR avoids materializing the contents of a file that was dragged
into an egui application on the web. It does so by storing the
`web_sys::File` handle directly on WASM.

This breaks the existing API of `DroppedFile` on the web, because there
is no way to retrieve the bytes synchronously form a `DroppedFile`
anymore, forcing handling call sites to become asynchronous.

The native API remains the same.

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-winit/src/dropped_file.rs

@@ -0,0 +1,22 @@
+use std::path::{Path, PathBuf};
+
+#[derive(Debug)]
+pub(crate) struct NativeFile {
+    path: PathBuf,
+}
+
+impl From<PathBuf> for NativeFile {
+    fn from(path: PathBuf) -> Self {
+        Self { path }
+    }
+}
+
+impl egui::DroppedFile for NativeFile {
+    fn path(&self) -> &Path {
+        &self.path
+    }
+
+    fn bytes(&self) -> Result<Vec<u8>, String> {
+        std::fs::read(&self.path).map_err(|err| err.to_string())
+    }
+}

crates/egui-winit/src/lib.rs

@@ -21,6 +21,7 @@ use egui::{Pos2, Rect, Theme, Vec2, ViewportBuilder, ViewportCommand, ViewportId
 pub use winit;
 
 pub mod clipboard;
+mod dropped_file;
 mod safe_area;
 mod window_settings;
 
@@ -28,6 +29,8 @@ pub use window_settings::WindowSettings;
 
 use raw_window_handle::HasDisplayHandle;
 
+use dropped_file::NativeFile;
+
 use winit::{
     dpi::{PhysicalPosition, PhysicalSize},
     event::ElementState,
@@ -470,10 +473,9 @@ impl State {
             }
             WindowEvent::DroppedFile(path) => {
                 self.egui_input.hovered_files.clear();
-                self.egui_input.dropped_files.push(egui::DroppedFile {
-                    path: Some(path.clone()),
-                    ..Default::default()
-                });
+                self.egui_input
+                    .dropped_files
+                    .push(std::sync::Arc::new(NativeFile::from(path.clone())));
                 EventResponse {
                     repaint: true,
                     consumed: false,