Add parameters to weston_fade_run() for setting the initial and target
values for the fade, as well as a parameter to set the spring constant
used for the animation.
Also add the weston_fade_update() function, that allows the animation
to be changed while it is still running.
This will be used to move the fade animation from core Weston into the
shell. These changes are needed to be able to fade out as well as in,
and to be able to reverse the fade in case of user input.
Ander Conselvan de Oliveira