Wait for the result at the target index if the future
is running and the iterator index is past the current
result count.
Determine if there is a result at the target index
after waitForResult() returns, and return -1/end if
not.
Also support decrementing the end iterator. In this
case wait for the future to finish in order to get
the final result count.
Task-number: QTBUG-59811
Change-Id: I8fcc711bab2e72c3c5196a55b794d25e18bb324d
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
6faa4d4a87