Support withdrawal of shadows

The Oxygen deco uses this to remove built-in shadows from decorated windows when adding (legacy...) decoration shadows

CCBUG: 291774
REVIEW: 103751
(cherry picked from commit a97bdc64fa2a676f99738abd371a99126fb8e93d)

shadow.cpp

@@ -206,6 +206,9 @@ bool Shadow::updateShadow()
 {
     QVector<long> data = Shadow::readX11ShadowProperty(m_topLevel->window());
     if (data.isEmpty()) {
+        if (m_topLevel && m_topLevel->effectWindow() && m_topLevel->effectWindow()->sceneWindow())
+            m_topLevel->effectWindow()->sceneWindow()->updateShadow(0);
+        deleteLater();
         return false;
     }
     init(data);