protocol: remove implicit attach semantics

To match the Weston commit e7144fd175d1d68b91aa0cec7ab63381b79385a9:
Author: Kristian Høgsberg <krh@bitplanet.net>
Date:   Mon Mar 4 12:11:41 2013 -0500

    compositor: Only send release event in response to wl_surface.attach

Remove the implicit attach semantics from wl_surface.commit and .attach.

Before, if you did this on a wl_surface: attach, commit, commit, you
would receive wl_buffer.release for both commits. After this change, you
will only receive wl_buffer.release for the first commit. To get a
second release, the same buffer must be attached again.

There is no need for the implicit attach on the second commit. If the
compositor needs the wl_buffer for repainting, it will not release it to
begin with. If the compositor does not need to keep the wl_buffer around
for repainting, it will not need it for a new commit either.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>