Fixing the .center() property

Now it is working properly, I have implemented it the right way now.

color.d

@@ -1509,6 +1509,11 @@ struct Rectangle {
 		return Point(right, bottom);
 	}
 
+	///
+	@property Point center() {
+		return Point((right + left) / 2, (bottom + top) / 2);
+	}
+
 	///
 	@property Size size() {
 		return Size(width, height);