As documented in cogl-pipeline-private.h, there is a precedence to the
ClutterPaintVolume bitfields that should be considered whenever we
implement code that manipulates PaintVolumes...

Firstly if ->is_empty == TRUE then the values for ->is_complete and
->is_2d are undefined, so we should typically check ->is_empty as the
first priority.

This fixes a bug in _clutter_paint_volume_cull() whereby we were
checking pv->is_complete before checking pv->is_empty which was
resulting in assertions for actors with no size.