desktop-shell: Make backgrounds fully opaque

Non-opaque backgrounds don't make sense, and cause rendering
problems.

Silently set backgrounds fully opaque, and remove any mention of
alpha from the man page.

Kiosk-shell already implicitly forces opaque backgrounds.

Old weston.ini with FF for alpha will always continue to work.

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