Implicit size for shapes

While this is not an issue specific to the curve renderer, it pops up regularly when writing tests and examples: Currently a Shape item has size 0x0, meaning that anchoring does not work out of the box.

In order for layouts etc. to work you have to manually set the size to match that of the bounding rect of the shape, applying transformations, accounting for the bounding rect's offset from its origin, etc.

For other Qt Quick items such as e.g. Text, it will have an implicit size which is used when nothing else is set. Adapting the same in Shape will make it a lot more convenient to use.