When calling QQuickWindow::update() on the GUI thread, we would
immediately flag the render thread that a repaint was needed and then
schedule a polishAndSync. If the render thread completed the current
frame before the GUI thread got to polishAndSync, it would repaint
right away without syncing with GUI first.

Instead of sending the repaint right away, register the need for a
full repaint in the window and set it as part of the next sync phase.

Change-Id: Ia731fb46724cc79f5391422213b069de9362d002
Reviewed-by: Giulio Camuffo <giulio.camuffo@jollamobile.com>
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>