Fixed canvas double drawing and made BackgroundColor virtual

Xwt.XamMac/Xwt.Mac/CanvasBackend.cs

@@ -52,6 +52,8 @@ protected override void OnSizeToFit ()
 			Widget.SetFrameSize (new CGSize ((nfloat)s.Width, (nfloat)s.Height)); 
 		}
 
+		public override Xwt.Drawing.Color BackgroundColor { get; set; }
+
 		public Rectangle Bounds {
 			get {
 				return new Rectangle (0, 0, view.Frame.Width, view.Frame.Height);

Xwt/Xwt/FrameBox.cs

@@ -223,7 +223,12 @@ public double BorderWidthBottom {
 			}
 		}
 
-		public Color BorderColor {
+        public override Color BackgroundColor {
+			get { return canvas.BackgroundColor; }
+			set { canvas.BackgroundColor = value; }
+		}
+
+        public Color BorderColor {
 			get { return borderColor; }
 			set { borderColor = value; canvas.QueueDraw (); }
 		}

Xwt/Xwt/Widget.cs

@@ -764,7 +764,7 @@ public Font Font {
 		/// Gets or sets the background color.
 		/// </summary>
 		/// <value>The background color of the widget.</value>
-		public Color BackgroundColor {
+		public virtual Color BackgroundColor {
 			get { return Backend.BackgroundColor; }
 			set { Backend.BackgroundColor = value; }
 		}