macOS: Ignore update requests for offline displays

Accessing a display that's offline may cause crashes,
so to be on the safe side we skip update requests in
this situation.

Task-number: QTBUG-102021
Pick-to: 6.2 6.3 6.4 5.15
Change-Id: I6b48b6722bccde628e510c538943d14f2b0271e5
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/platforms/cocoa/qcocoascreen.mm

@@ -275,6 +275,11 @@ void QCocoaScreen::requestUpdate()
 {
     Q_ASSERT(m_displayId);
 
+    if (!isOnline()) {
+        qCDebug(lcQpaScreenUpdates) << this << "is not online. Ignoring update request";
+        return;
+    }
+
     if (!m_displayLink) {
         CVDisplayLinkCreateWithCGDisplay(m_displayId, &m_displayLink);
         CVDisplayLinkSetOutputCallback(m_displayLink, [](CVDisplayLinkRef, const CVTimeStamp*,