Connect to ClutterStage::after-paint with connect_after()

Use connect_after() to accomodate code in GNOME Shell that,
when benchmarking drawing performance, connects to ::after-paint
and calls glFinish(). The timing information from that will be
more accurate if we hold off until that completes before we signal
apps to begin drawing the next frame. If there are no other
connections to ::after-paint, connect() vs. connect_after() doesn't
matter.

https://bugzilla.gnome.org/show_bug.cgi?id=732350