Summary:
If a shadow is installed for already rendered window, the shadow won't
be initially rendered because we don't rebuild window quad cache.

BUG: 398572
FIXED-IN: 5.14.0

Test Plan: Can't reproduce 398572 anymore.

Reviewers: #kwin, davidedmundson

Reviewed By: #kwin, davidedmundson

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D15475