[effects/slidingpopups] Simplify math in setupAnimData

Summary:
Also, this change fixes "possible bug" when sanitizing offset for
East/South(Right/Bottom) location.

QHash abuse will be fixed in the follow-up patches.

Reviewers: KWin, mart, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: davidedmundson, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D14088