[effects/magiclamp] Eliminate unnecessary reallocations

Summary:
The maximum size of newQuads is known in advance, so we can avoid
unnecessary reallocations when adding transformed window quads to
newQuads.

Test Plan: Minimized/unminimized System Settings.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14117