QQuickPaintedItem draws poorly on Retina displays, contains no support for 2x devicePixelRatio

QQuickPaintedItem draws poorly on Retina displays due to lack of support for 2x devicePixelRatio. The result is fuzzy draw.

Please see the attached files for examples of the fuzzy draw in comparison with and an improved draw using a provisional fix implemented by a colleague. Also, the provisional fix to qsgpainternode.cpp is attached.

BTW, our application contains a mix of QtQuick/QML and some additional classes which implement graphs which inherit from QQuickPaintedItem. We need QQuickPaintedItems to draw well on Retina displays, because our customers' target device may often be an iPad (3 or 4 current iPad models ship with Retina displays as stock) or MacBook Pro with Retina display (2 of 3 current MacBook Pro models ship with Retina display as stock).