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/emath/src/rect_align.rs

@@ -236,7 +236,7 @@ impl RectAlign {
     }
 
     /// Look for the first alternative [`RectAlign`] that allows the child rect to fit
-    /// inside the `screen_rect`.
+    /// inside the `content_rect`.
     ///
     /// If no alternative fits, the first is returned.
     /// If no alternatives are given, `None` is returned.
@@ -246,7 +246,7 @@ impl RectAlign {
     /// - [`RectAlign::MENU_ALIGNS`] for the 12 common menu positions
     pub fn find_best_align(
         values_to_try: impl Iterator<Item = Self>,
-        screen_rect: Rect,
+        content_rect: Rect,
         parent_rect: Rect,
         gap: f32,
         expected_size: Vec2,
@@ -258,7 +258,7 @@ impl RectAlign {
 
             let suggested_popup_rect = align.align_rect(&parent_rect, expected_size, gap);
 
-            if screen_rect.contains_rect(suggested_popup_rect) {
+            if content_rect.contains_rect(suggested_popup_rect) {
                 return Some(align);
             }
         }