[libkwineffects] Use std::copy

Summary: Don't use raw loop, use an algorithm from STL to copy effect pairs.

Test Plan: Effects are still working.

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14004