wasm: Apply window mask during QWasmWindow initialization

We need to apply properties of the QWindow the underlying "native
window", in this case our <div>, on construction, without waiting
for the user to call one of the QWindow setters.

Pick-to: 6.6
Change-Id: Id422a9424f584e2269ef333e2c7c88a123ecb70b
Reviewed-by: Mikołaj Boc <Mikolaj.Boc@qt.io>
Reviewed-by: Piotr Wierciński <piotr.wiercinski@qt.io>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/wasm/qwasmwindow.cpp

@@ -5,6 +5,7 @@
 #include <private/qguiapplication_p.h>
 #include <QtCore/qfile.h>
 #include <QtGui/private/qwindow_p.h>
+#include <QtGui/private/qhighdpiscaling_p.h>
 #include <private/qpixmapcache_p.h>
 #include <QtGui/qopenglfunctions.h>
 #include <QBuffer>
@@ -218,6 +219,8 @@ void QWasmWindow::initialize()
     setWindowState(window()->windowStates());
     setWindowFlags(window()->flags());
     setWindowTitle(window()->title());
+    setMask(QHighDpi::toNativeLocalRegion(window()->mask(), window()));
+
     if (window()->isTopLevel())
         setWindowIcon(window()->icon());
     m_normalGeometry = rect;