Load Kwin's internal cursors for the highest resolution of attached monitors

Test Plan:
Hovered over decoration
Looked super crystal clear
Same physical size as when I hover over window contents (which had a buffer scale of 1)

Reviewers: KWin, graesslin

Reviewed By: KWin, graesslin

Subscribers: graesslin, kwin

Tags: KWin

Differential Revision: https://phabricator.kde.org/D13608