Fix multimonitor blur

Summary:
On wayland blur on secondary monitor would not render correctly.

BUG: 393723
Depends on D12452

Test Plan:

• use more than one output
• log in in a wayland session
• open a transparent window (for example: Konsole with transparent and blur enabled profile)
• drag the window to another screen
• blurs the content under the window corretly

Reviewers: KWin, graesslin, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: apol, zzag, davidedmundson, kwin, KWin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D12678