QgsMapRendererJob and subclasses are not designed to be subclassed
outside of core QGIS code. Marking them private API allows us to
change them after API is frozen again.
Rationale:
- there was a lot of large objects passed by value, so potentially
there's a speed bump from this
- even for implicitly shared classes like QString/QList there's still
a (small) cost for copying the objects when there's no reason to
- it's the right thing to do!
