The API `wl_buffer_add_listener` is misleading in the sense that there
can be only one `wl_buffer` release callback, and trying to add a new
listener when once is already in place will lead to a protocol error.

The Xwayland EGL backends may need to set up their own `wl_buffer`
release listener, meaning that there is no way to our own `wl_buffer`
release callback.

To avoid the problem, add our own callback API to be notified when the
`wl_buffer` associated with an `xwl_pixmap` is released, triggered from
the different `xwl_pixmap` implementations.

Also update the Present code to use the new buffer release callback API.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>