[gnome-shell] background: Fix math to calculate the interval from the duration

[Jasper St. Pierre <jstpierre src gnome org>]

commit eeea8559b69e1cae87f89d6dd60ed8d2ef5deb51
Author: Jasper St. Pierre <jstpierre mecheye net>
Date:   Thu Mar 14 16:58:20 2013 -0400

    background: Fix math to calculate the interval from the duration
    
    The math here before was incorrect. This is still wrong, as we're
    looking at the total duration of the animation rather than the next
    step.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=695882

 js/ui/background.js |    5 ++++-
 1 files changed, 4 insertions(+), 1 deletions(-)
---
diff --git a/js/ui/background.js b/js/ui/background.js
index 9e3e009..bf8757e 100644
--- a/js/ui/background.js
+++ b/js/ui/background.js
@@ -442,8 +442,11 @@ const Background = new Lang.Class({
         if (!this._animation.duration)
             return;
 
+        let nSteps = 255 / ANIMATION_OPACITY_STEP_INCREMENT;
+        let timePerStep = (this._animation.duration * 1000) / nSteps;
+
         let interval = Math.max(ANIMATION_MIN_WAKEUP_INTERVAL * 1000,
-                                ANIMATION_OPACITY_STEP_INCREMENT / this._animation.duration);
+                                timePerStep);
         this._animationUpdateTimeoutId = GLib.timeout_add(GLib.PRIORITY_DEFAULT,
                                                       interval,
                                                       Lang.bind(this, function() {