Add a lazy version of QGeoPathPrivate

MapItems make heavy use of the contained geoshape
MapObjects do not.
Have a version of the shapes that eagerly compute the bounds and another that doesn't.

Also separate QGeoPathPrivate from QGeoPolygonPrivate.
No reason to carry useless baggage on classes that do not use it.

see https://codereview.qt-project.org/#/c/227858/