Return correct shape for Unmanaged clients

Summary:
Currently EffectWindowImpl::shape() falls back to the frame geometry
because isX11Client() returns invalid value for Unmanaged clients.

BUG: 415475

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D26542