shell: Add a new weston.ini to control the startup animation

The new key startup-animation in the [shell] section lets you
control the startup animation.  Default is fade, but with this patch
we can also do none, which just show the desktop as fast as possible.

man/weston.ini.man

@@ -150,8 +150,15 @@ enables screen locking (boolean).
 .BI "animation=" zoom
 sets the effect used for opening new windows (string). Can be
 .B zoom,
-.B fade.
-Otherwise, no animation is used.
+.B fade,
+.B none.
+By default, no animation is used.
+.TP 7
+.BI "startup-animation=" fade
+sets the effect used for opening new windows (string). Can be
+.B fade,
+.B none.
+By default, the fade animation is used.
 .TP 7
 .BI "binding-modifier=" ctrl
 sets the modifier key used for common bindings (string), such as moving