Make dispatcher more usable on non-ui threads

src/Avalonia.Base/Threading/AvaloniaSynchronizationContext.cs

@@ -99,14 +99,15 @@ public void Dispose()
             }
         }
 
-        public static RestoreContext Ensure(DispatcherPriority priority)
+        public static RestoreContext Ensure(DispatcherPriority priority) => Ensure(Dispatcher.UIThread, priority);
+        public static RestoreContext Ensure(Dispatcher dispatcher, DispatcherPriority priority)
         {
             if (Current is AvaloniaSynchronizationContext avaloniaContext 
                 && avaloniaContext.Priority == priority)
                 return default;
             var oldContext = Current;
-            Dispatcher.UIThread.VerifyAccess();
-            SetSynchronizationContext(Dispatcher.UIThread.GetContextWithPriority(priority));
+            dispatcher.VerifyAccess();
+            SetSynchronizationContext(dispatcher.GetContextWithPriority(priority));
             return new RestoreContext(oldContext);
         }
     }

src/Avalonia.Base/Threading/Dispatcher.Invoke.cs

@@ -106,7 +106,7 @@ public void Invoke(Action callback, DispatcherPriority priority, CancellationTok
         // call the callback directly.
         if (!cancellationToken.IsCancellationRequested && priority == DispatcherPriority.Send && CheckAccess())
         {
-            using (AvaloniaSynchronizationContext.Ensure(priority))
+            using (AvaloniaSynchronizationContext.Ensure(this, priority))
                 callback();
             return;
         }
@@ -228,7 +228,7 @@ public TResult Invoke<TResult>(Func<TResult> callback, DispatcherPriority priori
         // call the callback directly.
         if (!cancellationToken.IsCancellationRequested && priority == DispatcherPriority.Send && CheckAccess())
         {
-            using (AvaloniaSynchronizationContext.Ensure(priority))
+            using (AvaloniaSynchronizationContext.Ensure(this, priority))
                 return callback();
         }
 

src/Avalonia.Base/Threading/Dispatcher.MainLoop.cs

@@ -49,7 +49,7 @@ public void PushFrame(DispatcherFrame frame)
         try
         {
             _frames.Push(frame);
-            using (AvaloniaSynchronizationContext.Ensure(DispatcherPriority.Normal))
+            using (AvaloniaSynchronizationContext.Ensure(this, DispatcherPriority.Normal))
                 frame.Run(_controlledImpl);
         }
         finally

src/Avalonia.Base/Threading/DispatcherOperation.cs

@@ -258,7 +258,7 @@ internal void Execute()
 
         try
         {
-            using (AvaloniaSynchronizationContext.Ensure(Priority))
+            using (AvaloniaSynchronizationContext.Ensure(Dispatcher, Priority))
                 InvokeCore();
         }
         finally