desktop-shell: Add a placeholder curtain on new outputs

Desktop-shell currently waits for the desktop-shell client to set up a
background image. But the outputs are enabled before this happens, which
forces a repaint.

If the fade animation is enabled, there's a fade curtain in place, but if
it's disabled the scene graph is empty at the repaint.

This repaint with an empty scene graph can have very nasty consequences,
especially if frame buffer compression is in use, as the buffer isn't
rendered into and can be in an undefined state that confuses whatever
video hardware is trying to decode the buffer.

Add a temporary black curtain placeholder that ensures the scene graph has
something in it before the client gets a chance to commit the real
background.

Signed-off-by: Derek Foreman <derek.foreman@collabora.com>