evas: properly fix unref of ector renderer.

evas: properly fix unref of ector renderer.

Async rendering doesn't have a main loop cleanup function. The only one
being called is in the rendering thread. I wrongly assumed in my previous
patch that render_post on an object was called after the async render was
done which is obviously not the case as pointed by Subhransu. This patch
now wait for the async rendering to be done.