Extend whole-row Var evaluation to cope with the case that the sub-plan

Extend whole-row Var evaluation to cope with the case that the sub-plan
generating the tuples has resjunk output columns.  This is not possible for
simple table scans but can happen when evaluating a whole-row Var for a view.
Per example from Patryk Kordylewski.  The problem exists back to 8.0 but
I'm not going to risk back-patching further than 8.2 because of the many
changes in this area.