Fix rendering of the Qt platform example with high-dpi

- Provide a C++ platform event for the scale factor change and send it
- Report the correct logical and physical sizes to Slint

api/cpp/platform.rs

@@ -139,7 +139,7 @@ pub unsafe extern "C" fn slint_windowrc_has_active_animations(
     window_adapter.window().has_active_animations()
 }
 
-/// Dispatch a key pressed or release event
+/// Dispatch resize event
 #[no_mangle]
 pub unsafe extern "C" fn slint_windowrc_dispatch_resize_event(
     handle: *const WindowAdapterRcOpaque,
@@ -152,6 +152,18 @@ pub unsafe extern "C" fn slint_windowrc_dispatch_resize_event(
     });
 }
 
+/// Dispatch scale factor change event
+#[no_mangle]
+pub unsafe extern "C" fn slint_windowrc_dispatch_scale_factor_change_event(
+    handle: *const WindowAdapterRcOpaque,
+    scale_factor: f32,
+) {
+    let window_adapter = &*(handle as *const Rc<dyn WindowAdapter>);
+    window_adapter
+        .window()
+        .dispatch_event(i_slint_core::platform::WindowEvent::ScaleFactorChanged { scale_factor });
+}
+
 #[no_mangle]
 pub extern "C" fn slint_platform_update_timers_and_animations() {
     i_slint_core::platform::update_timers_and_animations()