Enable transitions to run on custom clocks.

src/Avalonia.Animation/Animatable.cs

@@ -69,7 +69,7 @@ namespace Avalonia.Animation
 
                 if (match != null)
                 {
-                    match.Apply(this, e.OldValue, e.NewValue);
+                    match.Apply(this, Clock.GlobalClock, e.OldValue, e.NewValue);
                 }
             }
         }

src/Avalonia.Animation/ITransition.cs

@@ -13,7 +13,7 @@ namespace Avalonia.Animation
         /// <summary>
         /// Applies the transition to the specified <see cref="Animatable"/>.
         /// </summary>
-        IDisposable Apply(Animatable control, object oldValue, object newValue);
+        IDisposable Apply(Animatable control, Clock clock, object oldValue, object newValue);
 
         /// <summary>
         /// Gets the property to be animated.

src/Avalonia.Animation/TransitionInstance.cs

@@ -18,10 +18,12 @@ namespace Avalonia.Animation
         private IDisposable timerSubscription;
         private TimeSpan startTime;
         private TimeSpan duration;
+        private readonly Clock _clock;
 
-        public TransitionInstance(TimeSpan Duration)
+        public TransitionInstance(Clock clock, TimeSpan Duration)
         {
             duration = Duration;
+            _clock = clock;
         }
 
         private void TimerTick(TimeSpan t)
@@ -45,8 +47,8 @@ namespace Avalonia.Animation
 
         protected override void Subscribed()
         {
-            startTime = Clock.GlobalClock.CurrentTime;
-            timerSubscription = Clock.GlobalClock.Subscribe(TimerTick);
+            startTime = _clock.CurrentTime;
+            timerSubscription = _clock.Subscribe(TimerTick);
             PublishNext(0.0d);
         }
     }

src/Avalonia.Animation/Transition`1.cs

@@ -49,9 +49,9 @@ namespace Avalonia.Animation
         public abstract IObservable<T> DoTransition(IObservable<double> progress, T oldValue, T newValue);
 
         /// <inheritdocs/>
-        public virtual IDisposable Apply(Animatable control, object oldValue, object newValue)
+        public virtual IDisposable Apply(Animatable control, Clock clock, object oldValue, object newValue)
         {
-            var transition = DoTransition(new TransitionInstance(Duration), (T)oldValue, (T)newValue);
+            var transition = DoTransition(new TransitionInstance(clock, Duration), (T)oldValue, (T)newValue);
             return control.Bind<T>((AvaloniaProperty<T>)Property, transition, Data.BindingPriority.Animation);
         }