Fix the constrain option not working in the window options demo (#8433)

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`constrain_to` overrides `constrain`, so I adjusted it to call
`constrain_to` before `constrain`.

* [x] I have followed the instructions in the PR template

Signed-off-by: limo520 <247564335@qq.com>

crates/egui_demo_lib/src/demo/window_options.rs

@@ -71,13 +71,13 @@ impl crate::Demo for WindowOptions {
         let mut window = egui::Window::new(title)
             .id(egui::Id::new("demo_window_options")) // required since we change the title
             .resizable(resizable)
+            .constrain_to(ui.available_rect_before_wrap())
             .constrain(constrain)
             .collapsible(collapsible)
             .movable(movable)
             .title_bar(title_bar)
             .drag_area(drag_area)
             .scroll(scroll2)
-            .constrain_to(ui.available_rect_before_wrap())
             .enabled(enabled);
         if closable {
             window = window.open(open);