[effects/wobblywindows] Draw clients below opaque wobbly windows

Summary:
Given that wobbly windows effect takes optimized render path, it needs
to clear the clip region of about to be transformed opaque window.

BUG: 411092
FIXED-IN: 5.17.0

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D23774