Avoid unnecessarily unwrapping stack_ref() after is_stack()

In a lot of cases, matching on `stack_ref()` also works.

src/wayland/handlers/compositor.rs

@@ -314,8 +314,7 @@ impl CompositorHandler for State {
                         .then(|| state.element())
                 });
             if let Some(window) = moved_window {
-                if window.is_stack() {
-                    let stack = window.stack_ref().unwrap();
+                if let Some(stack) = window.stack_ref() {
                     if let Some(i) = stack.surfaces().position(|s| {
                         s.wl_surface()
                             .as_deref()