compositor: Make sure we always call the post_paint handlers

Commit 4f2bb583 started to use a clutter_threads_add_repaint_func_full
callback instead of connecting to the stage's paint signal.

The callback has to return TRUE if it wants to be called again, so fix that
as we want to call it for every frame (otherwise apps supporting the WM SYNC
protocol will stop drawing).

https://bugzilla.gnome.org/show_bug.cgi?id=695006