To take screenshots, Enlightenment makes a new snapshot object, performs
a manual render, and uses the snapshot results.
Turns out if this happens while an async render is in progress, the
async render's completion triggers a render post callback on the snapshot
object even though it's never been involved in a render.
We need to defer new render post callbacks until any currently running
render completes, then add them during that render's post.
Fix T7156