Add safe area and document coordinate systems (#3890)

Added `Window::safe_area`, which describes the area of the surface that
is unobstructed by notches, bezels etc. The drawing code in the examples
have been updated to draw a star inside the safe area, and the plain
background outside of it.

Also renamed `Window::inner_position` to `Window::surface_position`, and
changed it to from screen coordinates to window coordinates, to better
align how these coordinate systems work together.

Finally, added some SVG images and documentation to describe how all of
this works.

This is fully implemented on macOS and iOS, and partially on the web.

Co-authored-by: daxpedda <daxpedda@gmail.com>

src/platform_impl/web/web_sys/safe_area.rs

@@ -0,0 +1,56 @@
+use dpi::{LogicalPosition, LogicalSize};
+use wasm_bindgen::JsCast;
+use web_sys::{Document, HtmlHtmlElement, Window};
+
+use super::Style;
+
+pub struct SafeAreaHandle {
+    style: Style,
+}
+
+impl SafeAreaHandle {
+    pub fn new(window: &Window, document: &Document) -> Self {
+        let document: HtmlHtmlElement = document.document_element().unwrap().unchecked_into();
+        #[allow(clippy::disallowed_methods)]
+        let write = document.style();
+        write
+            .set_property(
+                "--__winit_safe_area",
+                "env(safe-area-inset-top) env(safe-area-inset-right) env(safe-area-inset-bottom) \
+                 env(safe-area-inset-left)",
+            )
+            .expect("unexpected read-only declaration block");
+        #[allow(clippy::disallowed_methods)]
+        let read = window
+            .get_computed_style(&document)
+            .expect("failed to obtain computed style")
+            // this can't fail: we aren't using a pseudo-element
+            .expect("invalid pseudo-element");
+
+        SafeAreaHandle { style: Style { read, write } }
+    }
+
+    pub fn get(&self) -> (LogicalPosition<f64>, LogicalSize<f64>) {
+        let value = self.style.get("--__winit_safe_area");
+
+        let mut values = value
+            .split(' ')
+            .map(|value| value.strip_suffix("px").expect("unexpected unit other then `px` found"));
+        let top: f64 = values.next().unwrap().parse().unwrap();
+        let right: f64 = values.next().unwrap().parse().unwrap();
+        let bottom: f64 = values.next().unwrap().parse().unwrap();
+        let left: f64 = values.next().unwrap().parse().unwrap();
+        assert_eq!(values.next(), None, "unexpected fifth value");
+
+        let width = super::style_size_property(&self.style, "width") - left - right;
+        let height = super::style_size_property(&self.style, "height") - top - bottom;
+
+        (LogicalPosition::new(left, top), LogicalSize::new(width, height))
+    }
+}
+
+impl Drop for SafeAreaHandle {
+    fn drop(&mut self) {
+        self.style.remove("--__winit_safe_area");
+    }
+}