animation: also change stop value when updating a fade target

If we're fading a view with weston_fade_run() and half way through we
want to unfade it we can use weston_fade_update() to set a new alpha
target. When the animation finishes, reset_alpha() is called which
ensures the alpha value of the view is exactly as requested.

Although weston_fade_update() was updating the spring target so the
animation would look fine, it would then reset to the alpha target
given in the first call to weston_fade_run().

src/animation.c

@@ -330,6 +330,7 @@ WL_EXPORT void
 weston_fade_update(struct weston_view_animation *fade, float target)
 {
 	fade->spring.target = target;
+	fade->stop = target;
 }
 
 static void