embeddedwindows: Mask the painting manually by applying a clip

QWindow::setMask() is not guaranteed to turn the masked out areas
transparent, and it's up to the client to ensure this during
painting.

Change-Id: I1155b3ad095152a993532f2290cacb670e20daa7
Reviewed-by: Liang Qi <liang.qi@qt.io>
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>
(cherry picked from commit 5ce4aecd495667a50dd121930f9ca39dc07cfc36)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

tests/manual/embeddedwindows/main.cpp

@@ -33,6 +33,8 @@ protected:
     {
         QPainter painter(this);
         painter.setCompositionMode(QPainter::CompositionMode_Source);
+        if (!mask().isNull())
+            painter.setClipRegion(mask());
         painter.fillRect(QRect(0, 0, width(), height()),
             m_pressed ? QGradient(QGradient::JuicyPeach) : m_brush);
     }