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

src/animation.c

@@ -265,16 +265,19 @@ weston_fade_run(struct weston_surface *surface,
 					    fade_frame, done, data);
 
 	weston_spring_init(&fade->spring, k, start, end);
+
+	fade->spring.friction = 1400;
+	fade->spring.previous = -(end - start) * 0.03;
+
 	surface->alpha = start;
 
 	return fade;
 }
 
 WL_EXPORT void
-weston_fade_update(struct weston_surface_animation *fade,
+weston_fade_update(struct weston_surface_animation *fade, float target)
-		   float start, float end, float k)
 {
-	weston_spring_init(&fade->spring, k, start, end);
+	fade->spring.target = target;
 }
 
 static void

src/compositor.h

@@ -1142,8 +1142,7 @@ weston_fade_run(struct weston_surface *surface,
 		float start, float end, float k,
 		weston_surface_animation_done_func_t done, void *data);
 void
-weston_fade_update(struct weston_surface_animation *fade,
+weston_fade_update(struct weston_surface_animation *fade, float target);
-		   float start, float end, float k);
 
 struct weston_surface_animation *
 weston_slide_run(struct weston_surface *surface, float start, float stop,

src/shell.c

@@ -3135,12 +3135,11 @@ shell_fade(struct desktop_shell *shell, enum fade_type type)
 	}
 
 	if (shell->fade.animation)
-		weston_fade_update(shell->fade.animation,
+		weston_fade_update(shell->fade.animation, tint);
-				   shell->fade.surface->alpha, tint, 30.0);
 	else
 		shell->fade.animation =
 			weston_fade_run(shell->fade.surface,
-					1.0 - tint, tint, 30.0,
+					1.0 - tint, tint, 300.0,
 					shell_fade_done, shell);
 }
 
@@ -3457,7 +3456,7 @@ map(struct desktop_shell *shell, struct weston_surface *surface,
 	{
 		switch (shell->win_animation_type) {
 		case ANIMATION_FADE:
-			weston_fade_run(surface, 0.0, 1.0, 200.0, NULL, NULL);
+			weston_fade_run(surface, 0.0, 1.0, 300.0, NULL, NULL);
 			break;
 		case ANIMATION_ZOOM:
 			weston_zoom_run(surface, 0.8, 1.0, NULL, NULL);