client: process exposure updates in the main thread

This is important for when synchronous delivery is enabled as that should only happen from the main thread.

Change-Id: I524443c6d2dd8dee8d36834b6d3032eb8f8c1750
Reviewed-by: Vlad Zahorodnii <vlad.zahorodnii@kde.org>

src/plugins/platforms/wayland/qwaylandwindow.cpp

@@ -850,7 +850,7 @@ bool QWaylandWindow::waitForFrameSync(int timeout)
         qCDebug(lcWaylandBackingstore) << "Didn't receive frame callback in time, window should now be inexposed";
         mFrameCallbackTimedOut = true;
         mWaitingForUpdate = false;
-        updateExposure();
+        QMetaObject::invokeMethod(this, &QWaylandWindow::updateExposure, Qt::QueuedConnection);
     }
 
     return !mWaitingForFrameCallback;