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