desktop-shell: do not black out with startup "none"

desktop-shell: do not black out with startup "none"

Do not use a black blanket surface when the startup animation is
specified to be "none". This is the final fix needed to make the
screenshot test deterministic and independent of weston-desktop-shell.

Previously, the black surface would cover all outputs until
weston-desktop-shell signalled ready. Then, depending on the set
animation, either the black surface was immediately removed (none) or a
fade-in started (fade).

Now, when there is no black surface at all for "none", the compositor
will show garbage until weston-desktop-shell gets everything up. This
may be undesireable but works for tests. To have the old "none"
behaviour back, I would propose to add a new startup-animation value
"black" for it.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>