Fix the transitions bug by pushing zero immediately after Subscription.

Thanks to sir @grokys for this fix.

Fix XML Doc on Animatable.cs.

src/Avalonia.Animation/Animatable.cs

@@ -11,7 +11,7 @@ using Avalonia.Data;
 namespace Avalonia.Animation
 {
     /// <summary>
-    /// Base class for control which can have property transitions.
+    /// Base class for all animatable objects.
     /// </summary>
     public class Animatable : AvaloniaObject
     { 

src/Avalonia.Animation/TransitionsEngine.cs

@@ -37,7 +37,7 @@ namespace Avalonia.Animation
 
             PublishNext(interpVal);
         }
-  
+
         protected override void Unsubscribed()
         {
             timerSubscription?.Dispose();
@@ -49,6 +49,7 @@ namespace Avalonia.Animation
             timerSubscription = Timing
                                 .AnimationsTimer
                                 .Subscribe(t => TimerTick(t));
+            PublishNext(0.0d);
         }
     }
 }