[libkwineffects] Gracefully release previous window pixmap
Summary:
If the cross fade animation is cancelled, we are not gracefully
unreference the previous window pixmap.
This change addresses that issue by using RAII approach to
reference/unreference the previous window pixmap.
Test Plan: Manually. The Maximize and the Morphing Popups effect still work.
Reviewers: KWin, davidedmundson
Reviewed By: KWin, davidedmundson
Subscribers: davidedmundson, kwin
Tags: KWin
Differential Revision: https://phabricator.kde.org/D16391