[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