Fix scheduling repaints in Effect::prePaintScreen()

If a repaint is scheduled in the prePaintScreen() function, we want
it to be applied in the next frame, not the current one.

Currently, it doesn't work like this because prePaintScreen() runs first
then the Compositor gathers repaints and resets them.

This is important to qtquick effects that use qtquick3d as some items in
qtquick3d schedule repaints for the next frame after synchronizing, i.e.
in OffscreenQuickView::update() which is called in prePaintScreen() by
QuickSceneEffect.