On Raspberry Pi, weston-desktop-shell is so slow to start, that the
compositor has time to run the fade-in before the wallpaper is up. The
user launching Weston sees the screen flipping to black, the fbcon
fading in, and then the desktop popping up.

To fix this, wait for the weston-desktop-shell to draw
everything before starting the initial fade-in. A new request is
added to the private desktop-shell protocol to signal it. If a
desktop-shell client does not support the new request, the fade-in
happens already at bind time.

If weston-desktop-shell crashes, or does not send the 'desktop_ready'
request in 15 seconds, the compositor will fade in anyway. This should
avoid a blocked screen in case weston-desktop-shell malfunction.

shell_fade_startup() does not directly start the fade-in but schedules
an idle callback, so that the compositor can process all pending events
before starting the fade clock. Otherwise (on RPi) we risk skipping part
of the animation. Yes, it is a hack, that should have been done in
window.c and weston-desktop-shell instead.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>