wasm: Re-evaluate window flags when reparenting window

We apply CSS classes to the qt-window in setWindowFlags based on
whether it hasFrame(), hasBorder(), etc. Which in turn depends on
whether the window is a top level or child window.

As a QPlatformWindow may be created while a QWindow is top level,
and then reparented into another QPlatformWindow, we need to update
the window flags, and re-sync the CSS classes, as otherwise we end
up with child windows with shadows and tile bars.

Change-Id: Ibbb3d2230a880057053d7b12f93257d9b250aa6f
Reviewed-by: Lorn Potter <lorn.potter@gmail.com>
Reviewed-by: Piotr Wierciński <piotr.wiercinski@qt.io>
(cherry picked from commit e6db07c4e7cca41a94dd787d9a4938abab7918ac)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/plugins/platforms/wasm/qwasmwindow.cpp

@@ -742,6 +742,10 @@ void QWasmWindow::setMask(const QRegion &region)
 
 void QWasmWindow::setParent(const QPlatformWindow *)
 {
+    // The window flags depend on whether we are a
+    // child window or not, so update them here.
+    setWindowFlags(window()->flags());
+
     commitParent(parentNode());
 }