Add API to re-use a ShapePath between multiple shapes

When a shape is used in multiple places in the same scene, there is no way of sharing the rendering assets between these, meaning that a lot of duplicate work will be done for each item.

It is possible there is an opportunity for a general-purpose instancing Qt Quick API here (re-render the nodes of this item with this other matrix and these other properties), but at least for Shapes this is pretty common use case and would really benefit from not having to redo all the preprocessing for each item.