Support viewport-relative widget sizing

src/app.rs

@@ -594,6 +594,11 @@ pub trait App: 'static
 	/// physical dimensions.
 	fn on_resize( &mut self, _width: u32, _height: u32 ) {}
 
+	/// Called when the compositor reports a new integer buffer scale for the
+	/// main surface. The default is inert so apps that only care about physical
+	/// pixels can keep using [`on_resize`](Self::on_resize).
+	fn on_scale_changed( &mut self, _scale: u32 ) {}
+
 	/// Called once at startup with a channel sender that can be used from any
 	/// thread to deliver messages into the event loop.  Sending a message
 	/// immediately wakes the event loop — no polling delay.