Use QVector in paintGenericScreen

Summary:
Given that QList<T> will allocate items on the heap when sizeof(T) is
bigger than sizeof(void *), which is the case for Phase2Data, use
QVector instead.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D18198