On Wayland, add support for fractional scaling

This adds support for the fractional scaling on Wayland via the
wp-fractional-scale protocol.

Co-authored-by: Julian Orth <ju.orth@gmail.com>

src/window.rs

@@ -486,6 +486,7 @@ impl Window {
     /// ## Platform-specific
     ///
     /// - **X11:** This respects Xft.dpi, and can be overridden using the `WINIT_X11_SCALE_FACTOR` environment variable.
+    /// - **Wayland:** Uses the wp-fractional-scale protocol if available. Falls back to integer-scale factors otherwise.
     /// - **Android:** Always returns 1.0.
     /// - **iOS:** Can only be called on the main thread. Returns the underlying `UIView`'s
     ///   [`contentScaleFactor`].