Returns a function that builds semantic information for the picture drawn by this painter.
If the returned function is null, this painter will not contribute new
SemanticsNodes to the semantics tree and the CustomPaint corresponding
to this painter will not create a semantics boundary. However, if
CustomPaint.child is not null, the child may contribute SemanticsNodes
to the tree.
SemanticsBuilderCallback get semanticsBuilder => null;