ShaderEffectSource does not provide signal when content is updated

Currently ShaderEffectSource provides scheduleUpdate() method in its API which asynchronously causes the FBO content to be updated. However there does not seem to be a way to know when the content has been updated. This is especially awkward when using ShaderEffectSource as a recursive content source & destination, it is difficult to know when initial content has been rendered and feedback loop should be activated.

From the users point of view a timer does not seem to be a reliable way to workaround this, especially soon after the items have been created the update is sometimes delayed for a quite long periods.