+ * Gets the natural size of the given actor.
+ *
+ * If the actor is a Control, it returns the natural size of the Control. @see Control::GetNaturalSize()
+ * If the actor is an ImageActor, it returns the size of the image.
+ * If the actor is a TextActor, it returns the size of the text. @see Font::MeasureText()
+ * Otherwise it returns the actor's current size. @see Actor::GetCurrentSize()
+ *
+ * @param[in] actor The actor.