[effects/cubeslide] Cancel active animation when number of desktops has changed

Summary:
If a virtual desktop is removed, then desktopChanged will be followed by
numberDesktopsChanged signal. In which case, we have to cancel the
active animation because front_desktop might be no longer valid when
it's time to perform compositing.

BUG: 406452

Reviewers: KWin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D21064