Instead of possibly calling wl_surface_commit() out of
GdkFrameClock::after-paint, tick the transient parent clock so ::after-paint
can be eventually run.
This ensures that the subsurface coordinates (considered part of the state
of the parent) aren't committed untimely, and guaranteed to be orderly with
the wl_subsurface-relative state.
This is a gtk-side fix for https://bugzilla.gnome.org/show_bug.cgi?id=738887
af6eab850e