Fix default hotspot of a hidpi QCursor

The hotspot is defined in device independent coordinates, so
the default coordinates need to be divided by device pixel ratio.

Also, modify the scaling of cursor's pixmap to use SmoothTransformation to
generate cleaner looking lodpi cursors from hidpi cursors.

Change-Id: Ia938fd1e476e19e796f30712e23b06a5efed9964
Task-number: QTBUG-34116
Reviewed-by: Jake Petroules <jake.petroules@theqtcompany.com>