Support cursor scaling in X windowed backend

Test Plan:
Ran kwin_wayland --windowed  --scale2

Hovered over deco. Got massive and detailed cursor
Hovered over a wayland client (Qt 5.11 not dev)
Got a massive, but slightly blocky cursor

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: zzag, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13642