* Even if we don't have pyside2 to treat qwidgets as full objects and
access their methods and data, we still need to be able to pass around
the QWidget* as an opaque pointer to be able to use the API properly.
* This change falls back to just using SWIG's default opaque pointer
wrapping and unwrapping when pyside2 isn't available.
* This means that if we BlockInvoke a python callback, we don't end up
in a deadlock where we're holding the GIL during script execution but
also waiting on the renderer running (which is trying to acquire the
GIL before calling the callback).
baldurk