wayland.xml: Make releases for multiple 'wl_surface.attach' undefined

Fixes #46

The way wl_buffer is specified makes this situation inherently racy,
meaning there is no way this can be done unambiguously. Current real
compositor implementations already have differing behaviour for this, so
any client relying on it was already broken, if any such client exists.

This specifically only singles out wl_buffer.release as being undefined;
every other aspect of it should still be valid. This is so existing and
correct uses of multiple attaches are still valid, where a
"static"/immutable wl_buffer is being used (i.e. they don't care about
the release event).

Signed-off-by: Scott Anderson <scott.anderson@collabora.com>