the previous formulation creates an ndarray and then creates an QImage
over it w/o copying. this relies on the QImage thus created being
non-const.
if the QImage was (unintentionally) created as const, the subsequent
QPainter render on the const QImage would trigger a COW. i.e. the
original underlying ndarray wouldn't be updated.
this is only an issue for PyQt bindings where const QImages can be
created.
659392d4bb