[effects/minimizeanimation] Use interpolate helper
Summary:
That's not really "clear" what's going on with opacity values. Use
interpolate helper to improve a little bit readability.
Test Plan: Minimized/uniminized System Settings, still works as expected.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D14553