Comparison with field rather than parameter causes crash (#85)

* Bugfix: Must take polymorphic controls into consideration.

* Bugfix: Must save original position of all tagged controls, regardless of the primary drag vertex status.

Before the fix, it was possible to get out of sync with the "IsTagged" status of the primary drag vertex (it was set after the drag start). When that happened, the secondary vertices being dragged, did not have its OriginalX/Y properties updated, resulting in a "jump" from its visual origin to X=0, Y=0.

* Bugfix: compare with argument, not default initialized field.

GraphX.PCL.Logic/Algorithms/LayoutAlgorithms/Tree/SimpleTreeLayoutAlgorithm.cs

@@ -24,7 +24,7 @@ namespace GraphX.PCL.Logic.Algorithms.LayoutAlgorithms
         public SimpleTreeLayoutAlgorithm( TGraph visitedGraph, IDictionary<TVertex, Point> vertexPositions, IDictionary<TVertex, Size> vertexSizes, SimpleTreeLayoutParameters parameters )
             : base( visitedGraph, vertexPositions, parameters )
         {
-            VertexSizes = VertexSizes == null ? new Dictionary<TVertex, Size>() : new Dictionary<TVertex, Size>(vertexSizes);
+            VertexSizes = vertexSizes == null ? new Dictionary<TVertex, Size>() : new Dictionary<TVertex, Size>(vertexSizes);
         }
 
         public override void Compute(CancellationToken cancellationToken)