If the screensaver is explicitly asked to lock
(as opposed to locking because of idle), then
we should do it very quickly.  1 second is
too long.

This commit changes the fade to 60 steps per
second and a quarter of second long
(put another way, about 15 fade increments).

Taken from gnome-screensaver,
commit:44edc03104f9757f4d866f24df8793f064b6ce70
And applied to MATE by infirit