Add support for the safe area on iOS (#7578)

This PR is a continuation of #4915 by @frederik-uni and @lucasmerlin
that introduces support for keeping egui content within the 'safe area'
on iOS (avoiding the notch / dynamic island / menu bar etc.), with the
following changes:

- `SafeArea` now wraps `MarginF32` and has been renamed to
`SafeAreaInsets` to clarify its purpose.
- `InputState::screen_rect` is now marked as deprecated in favour of
either `viewport_rect` (which contains the entire screen), or
`content_rect` (which is the viewport rect with the safe area insets
removed).
- I added some comments to the safe area insets logic pointing out the
[safe area API coming in winit
v0.31](https://github.com/rust-windowing/winit/issues/3910).

---------

Co-authored-by: frederik-uni <147479464+frederik-uni@users.noreply.github.com>
Co-authored-by: Lucas Meurer <hi@lucasmerlin.me>
Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-winit/src/lib.rs

@@ -18,6 +18,7 @@ use egui::{Pos2, Rect, Theme, Vec2, ViewportBuilder, ViewportCommand, ViewportId
 pub use winit;
 
 pub mod clipboard;
+mod safe_area;
 mod window_settings;
 
 pub use window_settings::WindowSettings;
@@ -274,6 +275,21 @@ impl State {
         }
 
         use winit::event::WindowEvent;
+
+        #[cfg(target_os = "ios")]
+        match &event {
+            WindowEvent::Resized(_)
+            | WindowEvent::ScaleFactorChanged { .. }
+            | WindowEvent::Focused(true)
+            | WindowEvent::Occluded(false) => {
+                // Once winit v0.31 has been released this can be reworked to get the safe area from
+                // `Window::safe_area`, and updated from a new event which is being discussed in
+                // https://github.com/rust-windowing/winit/issues/3911.
+                self.egui_input_mut().safe_area_insets = Some(safe_area::get_safe_area_insets());
+            }
+            _ => {}
+        }
+
         match event {
             WindowEvent::ScaleFactorChanged { scale_factor, .. } => {
                 let native_pixels_per_point = *scale_factor as f32;