MetaWaylandSurface: Keep an extra buffer use count for role-less surfaces

Whether a surface needs to keep the committed wl_buffer un-released
depends on what role the surface gets assigned to. For example a cursor
role may need an unreleased shm buffer in order to create a hw cursor from
it.

In order to support this, keep a separate reference and use count to
the buffer on behalf of the in the future assigned role, and release
those references after the surface was assigned a role. A role that
needs its own references and use counts, must in its assign function
make sure to add those.

https://bugzilla.gnome.org/show_bug.cgi?id=762828