protocol: Change wording of subsurface placement scheduling

Change wording to be more consistent with other parts of the subsurface
protocol. Before this change, wl_subsurface.set_position explicitly
stated that the new state was to be applied on the parents
wl_surface.commit and wl_subsurface.place_above/below only said "on
the next commit of the parent surface". What "committed" means is
ambiguous considering that a wl_surface.commit actually defers the
actual commit when in synchronized mode, but the intention has always
been that placement of a subsurface should be considered part of its
content, i.e. placement state should be applied when other state
(buffer, regions). This patch makes that more clear.

Note that prior to this patch, one could correctly have interpreted
the protocol meaning that placements operations takes effect explicitly
on wl_surface.commit of the parent surface no matter whether other state
of the parent surface is applied at that point. This patch clarifies that
that is not the case.

https://bugs.freedesktop.org/show_bug.cgi?id=88857



Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Jasper St. Pierre <jstpierre@mecheye.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>