Summary:
We know that the maximum size is the number of windows in stacking
order. It might be that some are skipped as they are not ready for
painting, but overall it's better to reserve a little bit too much
memory than to have a growing QVector.

Test Plan: Compiles

Reviewers: #kwin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D18208