shouldRebuildSemantics method

Called whenever a new instance of the custom painter delegate class is provided to the RenderCustomPaint object, or any time that a new CustomPaint object is created with a new instance of the custom painter delegate class (which amounts to the same thing, because the latter is implemented in terms of the former).

If the new instance would cause semanticsBuilder to create different semantics information, then this method should return true, otherwise it should return false.

If the method returns false, then the semanticsBuilder call might be optimized away.

It's possible that the semanticsBuilder will get called even if shouldRebuildSemantics would return false. For example, it is called when the CustomPaint is rendered for the very first time, or when the box changes its size.

By default this method delegates to shouldRepaint under the assumption that in most cases semantics change when something new is drawn.