Interface: IRectangleJSON

What toJSON() returns: the placed rect's public shape (issue #53).

width, height, x, y and rot are always present. The rest are optional because they are only written when they carry something: oversized when the packer tagged the rect, allowRotation when the rect has an explicit override, data when the payload is not the constructor's {} placeholder. The index signature leaves room for a subclass's own fields, which are serialized as well.

The shape satisfies IRectangleInput, so the result of JSON.parse(JSON.stringify(rect)) can be handed straight back to addArray().