[effects] Don't use dynamic_cast
Summary:
qobject_cast has several advantages over dynamic_cast:
- it works without RTTI;
- it's usually faster.
Also, when it comes to casting QObjects, qobject_cast is de facto
standard.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D17427