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/macos.rs

@@ -92,6 +92,9 @@ pub trait WindowExtMacOS {
     /// This is how fullscreen used to work on macOS in versions before Lion.
     /// And allows the user to have a fullscreen window without using another
     /// space or taking control over the entire monitor.
+    ///
+    /// Make sure you only draw your important content inside the safe area so that it does not
+    /// overlap with the notch on newer devices, see [`Window::safe_area`] for details.
     fn set_simple_fullscreen(&self, fullscreen: bool) -> bool;
 
     /// Returns whether or not the window has shadow.