Set missing flags in the option when rendering QTreeView drag pixmap

Set missing flags in the option when rendering QTreeView drag pixmap

QAbstractItemViewPrivate::renderToPixmap was not setting all the flags
that the normal QTreeView painting sets:
option.showDecorationSelected, option.viewItemPosition (so the drag pixmap
looked wrong on Windows 7, with rects around each cell), and then the
unittest also discovered that State_Children/State_Sibling wasn't set either.

Task-number: QTBUG-15834
Merge-request: 2517
Reviewed-by: Gabriel