Advance window animations in prePaintWindow()

This will ensure that the window animation is advanced only if the
window is painted. I'd like to rely on this to allow effects schedule
animations as soon as Workspace::windowAdded signal is emitted.

As is, the issue with delaying EffectsHandler::windowAdded is that we
don't filter out all "not ready" windows, which breaks some effects
because EffectWindows are leaked in some properties but no
EffectsHandler::windowRemoved signal is emitted so those effects can
perform cleanup. Fixing that by the means of ensuring that only renderable
EffectWindows are exposed is hard too, and it would also mean that
effects need to use their own api, which I would like to avoid and port
the effects to kwin core apis instead.