[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: {F5828442}

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