Use new clutter_stage_set_paint_callback() function for after-paint notification

Commit 4f2bb583 changed things so that the compositor used
clutter_threads_add_repaint_func_full (CLUTTER_REPAINT_FLAGS_POST_PAINT
to get after-paint notification and send _NET_WM_FRAME_DRAWN, but this
doesn't actually work, since Clutter will already have blocked for
VBlank before calling post-paint functions.

The result is that frame synced toolkits like GTK 3.8 will normally
only be able to draw every other frame.

Since ::paint doesn't work either, a new function
clutter_stage_set_paint_callback() has been added to Clutter
(and will be included in the 1.14 branch)

https://bugzilla.gnome.org/show_bug.cgi?id=698794