shouldRepaint abstract method

Determines whether repaint is needed when a new RenderEditablePainter is provided to a RenderEditable.

If the new instance represents different information than the old instance, then the method should return true, otherwise it should return false. When oldDelegate is null, this method should always return true unless the new painter initially does not paint anything.

If the method returns false, then the paint call might be optimized away. However, the paint method will get called whenever the RenderEditables it attaches to repaint, even if shouldRepaint returns false.