scene: Pass non-trivial classes by const&
Summary:
This will save the copy of some objects, especially PaintData classes that are
not copy-on-write.
It also follows the practice on other parts of the system.
Test Plan: Running it right now
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D28031