Added window::Id to multi_window application's scale_factor

winit/src/multi_window.rs

@@ -146,7 +146,7 @@ where
     /// while a scale factor of `0.5` will shrink them to half their size.
     ///
     /// By default, it returns `1.0`.
-    fn scale_factor(&self) -> f64 {
+    fn scale_factor(&self, window: window::Id) -> f64 {
         1.0
     }