[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