[effects/presentwindows] Enable blur behind windows

Summary:
Use the blur effect even when the present windows is used.
Previously it was only enabled for the close buttons and the dock.

Test Plan:

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

Reviewed By: VDG, KWin, graesslin, ngraham

Subscribers: ngraham, zzag, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12619