[effects/desktopgrid] Enable blur behind windows

Summary: Use the blur effect even when the desktop grid is used.

Test Plan:

Reviewers: davidedmundson, fredrik, VDG, KWin, graesslin, ngraham

Reviewed By: VDG, KWin, graesslin, ngraham

Subscribers: graesslin, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12620