paint-volumes: remove alignment constraint for completion

paint-volumes: remove alignment constraint for completion

This removes the constraint that a paint-volume must be axis aligned
before _clutter_paint_volume_complete can be called. NB: A paint volume
is represented by one origin vertex and then three axis vertices to
define the width, height and depth of the volume. It's straightforward
to use the vectors from the origin to the axis vertices to deduce the
other 4 vertices so we can remove the is_axis_aligned assertion.