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