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