The current semantics would implicitly attach the most recently attached
buffer at commit time and send a release event when the buffer enventually
was released.  The implicit attach is a little too subtle though and this
patch changes the semantics to always only send release events in response
to an attach event.  As a consequence, once a compositor releases a buffer,
it no longer has a reference to it and wl_surfcea.damage is undefined.
Thus, the client side visible change is that damage request must always
be preceeded by a wl_surface.attach request, to ensure there's a valid buffer,
even if that means attaching the same buffer again.