libweston/screnshooter: Fix weston screenshot event done if there's no client running

The 'done' event sent back to client with the weston screenshot interface
is not being sent if there is no damage on the plane. This patch (re-uses just
like recording part) weston_output_damage() to achieve that.

Otherwise the client will have to wait (and be blocked) until some
damage on the plane is being done.

Signed-off-by: Marius Vlad <marius.vlad0@gmail.com>