When many runnables are executed, this improves the
performance by not resizing the queue for each runnable,
which was the case in the previous version, because of
many calls to QVector::takeFirst().
Also add a test that makes sure tryTake() is safe to
call and does not leave the queue in a bad state that
tries to use nullptr entries.
Change-Id: I608134ecfa9cfc03db4878dcbd6f9c1107e13e90
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
ba423261cd