Correctly process KeyTime to Cue by using TimeSpan.TotalSeconds instead of Ticks.

src/Avalonia.Animation/Animation.cs

@@ -251,7 +251,7 @@ namespace Avalonia.Animation
 
                     if (keyframe.TimingMode == KeyFrameTimingMode.TimeSpan)
                     {
-                        cue = new Cue(keyframe.KeyTime.Ticks / Duration.Ticks);
+                        cue = new Cue(keyframe.KeyTime.TotalSeconds / Duration.TotalSeconds);
                     }
 
                     var newKF = new AnimatorKeyFrame(handler, cue);