[effects/slideback] Enable blur behind windows

Summary: Use the blur effect even while the window is sliding back.

Test Plan:

Reviewers: KWin, VDG, graesslin, ngraham

Reviewed By: KWin, VDG, graesslin, ngraham

Subscribers: ngraham, graesslin, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12625