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>

tests/test_viewports/src/main.rs

@@ -247,8 +247,12 @@ fn generic_ui(ui: &mut egui::Ui, children: &[Arc<RwLock<ViewportState>>], close_
         if let Some(monitor_size) = ctx.input(|i| i.viewport().monitor_size) {
             ui.label(format!("monitor_size: {monitor_size:?} (points)"));
         }
-        if let Some(screen_rect) = ui.input(|i| i.raw.screen_rect) {
-            ui.label(format!("Screen rect size: Pos: {:?}", screen_rect.size()));
+        if let Some(viewport_rect) = ui.input(|i| i.raw.screen_rect) {
+            ui.label(format!(
+                "Viewport Rect: Pos: {:?}, Size: {:?} (points)",
+                viewport_rect.min,
+                viewport_rect.size()
+            ));
         }
         if let Some(inner_rect) = ctx.input(|i| i.viewport().inner_rect) {
             ui.label(format!(