DRM cursor scaling

Summary:
We pass the buffer scale of the cursor through QImage::devicePixelRatio.

When copying the cursor with QPainter use Qt's in-built functionality to
handle resizing the cursor pixmap as necessary to match the screen.

As we're now resizing the cursor, the hotspot needs translating from
logical to device co-ordinates.

Test Plan: Used with associated patches

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D13605