desktop-shell: Don't set false to is_mapped of view when fade out the view finished

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 leaving is_mapped true, the output repaint scheduled without the surface.

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