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>
2 jobs for desktop-shell-fix-fade-out in 59 seconds (queued for 1 second)
Status Job ID Name Coverage
  Container Prep
passed #350503
container_prep

00:00:28

 
  Build
passed #350504
build-native-meson

00:00:31