desktop-shell: unmap a view which was faded out

When Fading out a destroyed surface view finishes, the view is rendered
with very little alpha. After that, since the output isn't updated
unless a event on the output doesn't occurs, the view is still on the
output. By unmapping the view, the output repaint scheduled without the
surface.

Signed-off-by: Tomohito Esaki <etom@igel.co.jp>