message: Add `UNIT` constant to `Size`
diff: @@ -15,6 +15,11 @@ impl Size {
  /// [`Size`]: struct.Size.html
  pub const ZERO: Size = Size::new(0., 0.);
  + /// A [`Size`] with a width and height of 1 unit.
  + ///
  + /// [`Size`]: struct.Size.html
  + pub const UNIT: Size = Size::new(1., 1.);
  +
  /// A [`Size`] with infinite width and height.
  ///
  /// [`Size`]: struct.Size.html
  