Maintain z-ordering of paths

When a Shape has multiple path objects, and one of them is modified, it moves to the front. I.e. the ordering of the curve node children is not maintained by updateNode().

Can be easily tested on the Interactive Shape tab in the public shapes example: create two filled and stroked curves, one overlapping the other, then modify the one behind. The other renderers maintain the z-ordering, CurveRenderer does not.