spring: Tweak the window fade-in spring to not overshoot

The fade animation overshoots and rings for a bit dipping back into
slightly transparent.  Bug 61675 was right.

https://bugs.freedesktop.org/show_bug.cgi?id=61675
diff --git a/src/compositor.h b/src/compositor.h
index faa2f20..4c746ca 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -1142,8 +1142,7 @@
 		float start, float end, float k,
 		weston_surface_animation_done_func_t done, void *data);
 void
-weston_fade_update(struct weston_surface_animation *fade,
-		   float start, float end, float k);
+weston_fade_update(struct weston_surface_animation *fade, float target);
 
 struct weston_surface_animation *
 weston_slide_run(struct weston_surface *surface, float start, float stop,