Add node description to painted items.

Change-Id: I47ad9df1550649131ac5379acbf7d133088c60a8
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Initialize QSGPaintedNode with an empty size.

Failing to do so would cause it to not update its size when set to 1x1
which in turn would cause it to never update its texture, causing crashes
later during rendering.

Task-number: QTBUG-39555
Change-Id: I099d9ef2d6968ece21f7c03819abfd6685bec430
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>