Replace foreach loops and improve code style in composite

Summary: Replaces foreach loops with modern for loops and improve code style overall.

Test Plan: Auto tests pass as before and manually in X and Wayland sessions.

Reviewers: KWin

Subscribers: zzag, kwin

Tags: KWin

Maniphest Tasks: T11071

Differential Revision: https://phabricator.kde.org/D23011