Window-functions patch handling of aggregates

The window functions patch is laboring under the delusion that it can
call an aggregate's final function and then go back to invoking the
transfn some more on the same data. This is merest fantasy :-(

regression=# select array_agg(q1) over(order by q1) from int8_tbl;
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.

Unless we want to move the goalposts on what an aggregate is allowed
to do internally, we're going to have to change this to re-aggregate
repeatedly. Neither prospect is appetizing in the least.

regards, tom lane