Prevent asserts in certain QWindow re-creation cases

Amends 402efef57b. The original patch
has a problem, namely that it directly calls QPlatformWindow::requestUpdate()
instead of going through QWindow::requestUpdate(). As there is a chance that
an update gets scheduled between the creation of the QPlatformWindow and this
extra, optional invocation of requestUpdate() at the end of QWindow::create(),
this becomes quite unsafe because QPlatformWindow, unlike QWindow, is not
graceful: it will just assert if there is a pending update still.

Solve the whole thing by storing the updateRequestPending flag of QWindowPrivate,
then resetting it, and then going through the safe, public
QWindow::requestUpdate() when our copy of the flag says so.

Task-number: QTBUG-81400
Task-number: QTBUG-70957
Pick-to: 5.15
Change-Id: I99aedfae3928b75301b46a4666c169e657ff8079
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qwindow.cpp

@@ -515,6 +515,11 @@ void QWindowPrivate::create(bool recursive, WId nativeHandle)
     if (platformWindow)
         return;
 
+    // avoid losing update requests when re-creating
+    const bool needsUpdate = updateRequestPending;
+    // the platformWindow, if there was one, is now gone, so make this flag reflect reality now
+    updateRequestPending = false;
+
     if (q->parent())
         q->parent()->create();
 
@@ -553,8 +558,8 @@ void QWindowPrivate::create(bool recursive, WId nativeHandle)
     QPlatformSurfaceEvent e(QPlatformSurfaceEvent::SurfaceCreated);
     QGuiApplication::sendEvent(q, &e);
 
-    if (updateRequestPending)
-        platformWindow->requestUpdate();
+    if (needsUpdate)
+        q->requestUpdate();
 }
 
 void QWindowPrivate::clearFocusObject()