Set correct DPR on wayland cursors received from remote buffers

Summary:
It will then be renderered appropriately when painting to the output
buffer.

Test Plan: Updated unit test, plus used with other relevant patches

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13606