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>

dpi/src/lib.rs

@@ -759,6 +759,150 @@ impl<P: Pixel> From<LogicalPosition<P>> for Position {
     }
 }
 
+/// The logical distance between the edges of two rectangles.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Default, Hash)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub struct LogicalInsets<P> {
+    /// The distance to the top edge.
+    pub top: P,
+    /// The distance to the left edge.
+    pub left: P,
+    /// The distance to the bottom edge.
+    pub bottom: P,
+    /// The distance to the right edge.
+    pub right: P,
+}
+
+impl<P> LogicalInsets<P> {
+    #[inline]
+    pub const fn new(top: P, left: P, bottom: P, right: P) -> Self {
+        Self { top, left, bottom, right }
+    }
+}
+
+impl<P: Pixel> LogicalInsets<P> {
+    #[inline]
+    pub fn from_physical<T: Into<PhysicalInsets<X>>, X: Pixel>(
+        physical: T,
+        scale_factor: f64,
+    ) -> Self {
+        physical.into().to_logical(scale_factor)
+    }
+
+    #[inline]
+    pub fn to_physical<X: Pixel>(&self, scale_factor: f64) -> PhysicalInsets<X> {
+        assert!(validate_scale_factor(scale_factor));
+        let top = self.top.into() * scale_factor;
+        let left = self.left.into() * scale_factor;
+        let bottom = self.bottom.into() * scale_factor;
+        let right = self.right.into() * scale_factor;
+        PhysicalInsets::new(top, left, bottom, right).cast()
+    }
+
+    #[inline]
+    pub fn cast<X: Pixel>(&self) -> LogicalInsets<X> {
+        LogicalInsets {
+            top: self.top.cast(),
+            left: self.left.cast(),
+            bottom: self.bottom.cast(),
+            right: self.right.cast(),
+        }
+    }
+}
+
+/// The physical distance between the edges of two rectangles.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Default, Hash)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub struct PhysicalInsets<P> {
+    /// The distance to the top edge.
+    pub top: P,
+    /// The distance to the left edge.
+    pub left: P,
+    /// The distance to the bottom edge.
+    pub bottom: P,
+    /// The distance to the right edge.
+    pub right: P,
+}
+
+impl<P> PhysicalInsets<P> {
+    #[inline]
+    pub const fn new(top: P, left: P, bottom: P, right: P) -> Self {
+        Self { top, left, bottom, right }
+    }
+}
+
+impl<P: Pixel> PhysicalInsets<P> {
+    #[inline]
+    pub fn from_logical<T: Into<LogicalInsets<X>>, X: Pixel>(
+        logical: T,
+        scale_factor: f64,
+    ) -> Self {
+        logical.into().to_physical(scale_factor)
+    }
+
+    #[inline]
+    pub fn to_logical<X: Pixel>(&self, scale_factor: f64) -> LogicalInsets<X> {
+        assert!(validate_scale_factor(scale_factor));
+        let top = self.top.into() / scale_factor;
+        let left = self.left.into() / scale_factor;
+        let bottom = self.bottom.into() / scale_factor;
+        let right = self.right.into() / scale_factor;
+        LogicalInsets::new(top, left, bottom, right).cast()
+    }
+
+    #[inline]
+    pub fn cast<X: Pixel>(&self) -> PhysicalInsets<X> {
+        PhysicalInsets {
+            top: self.top.cast(),
+            left: self.left.cast(),
+            bottom: self.bottom.cast(),
+            right: self.right.cast(),
+        }
+    }
+}
+
+/// Insets that are either physical or logical.
+#[derive(Debug, Copy, Clone, PartialEq)]
+#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
+pub enum Insets {
+    Physical(PhysicalInsets<u32>),
+    Logical(LogicalInsets<f64>),
+}
+
+impl Insets {
+    pub fn new<S: Into<Self>>(insets: S) -> Self {
+        insets.into()
+    }
+
+    pub fn to_logical<P: Pixel>(&self, scale_factor: f64) -> LogicalInsets<P> {
+        match *self {
+            Self::Physical(insets) => insets.to_logical(scale_factor),
+            Self::Logical(insets) => insets.cast(),
+        }
+    }
+
+    pub fn to_physical<P: Pixel>(&self, scale_factor: f64) -> PhysicalInsets<P> {
+        match *self {
+            Self::Physical(insets) => insets.cast(),
+            Self::Logical(insets) => insets.to_physical(scale_factor),
+        }
+    }
+}
+
+impl<P: Pixel> From<PhysicalInsets<P>> for Insets {
+    #[inline]
+    fn from(insets: PhysicalInsets<P>) -> Self {
+        Self::Physical(insets.cast())
+    }
+}
+
+impl<P: Pixel> From<LogicalInsets<P>> for Insets {
+    #[inline]
+    fn from(insets: LogicalInsets<P>) -> Self {
+        Self::Logical(insets.cast())
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use std::collections::HashSet;