[effects/slideback] Enable blur behind windows

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

Test Plan: {F5828577}

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