Fix blur on Wayland when scaling is used

Summary:
Blur should now work on Wayland when scaling is used.
This does not affect X11 as `GLRenderTarget::virtualScreenScale()` is always 1 on X11

BUG: 391387
Depends on D12678

Test Plan:
  - log in in a Wayland session
  - turn display scaling to 2x
  - open a transparent window (for example: Konsole with transparent and blur enabled profile)
  - blurs the content under the window corretly

Reviewers: davidedmundson, #kwin

Reviewed By: davidedmundson, #kwin

Subscribers: romangg, apol, zzag, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D12700