Added Size.Empty

src/Perspex.SceneGraph/Size.cs

@@ -16,6 +16,11 @@ namespace Perspex
         /// </summary>
         public static readonly Size Infinity = new Size(double.PositiveInfinity, double.PositiveInfinity);
 
+        /// <summary>
+        /// A size representing zero
+        /// </summary>
+        public static readonly Size Empty = new Size(0, 0);
+
         /// <summary>
         /// The width.
         /// </summary>