Add a deferred clip preferences update.
The getClipPreferences action has now be separated in 2 passes: the
first pass to fetch the infos from the plug-in and a second pass to
update all the clips and the effect after Natron’s proxy has been
applied.
The update happens when all actions are terminated and cannot run
concurrently to any other action.

You can now get the effect’s preferred output aspect ratio for
rendering by calling getPreferredAspectRatio() in renderRoI()