compositor: Don't connect to the stage's paint signal

Doing so causes useless full stage redraws and breaks culling
as clutter cannot know how the signal handler affects painting.

So use clutter_threads_add_repaint_func_full with the
CLUTTER_REPAINT_FLAGS_POST_PAINT flag instead.

https://bugzilla.gnome.org/show_bug.cgi?id=694988