QTreeView drag pixmap rendering bug on Windows, due to missing style options

When dragging a QTreeView row with multiple columns, the drag pixmap looks wrong on Windows (at least Windows 7) : each column gets a blue rounded-rect in the background, rather than seeing a single rounded-rect around the whole row (like we can see in the actual treeview).

This is because QAbstractItemViewPrivate::renderToPixmap asks the delegate to draw the dragged indexes, but doesn't set option.viewItemPosition to Beginning/Middle/End.

To see the bug for yourself, just edit examples/itemviews/dirview/main.cpp and add tree.setDragDropMode(QAbstractItemView::DragOnly),
then drag a file on Windows, and look at the drag pixmap.

I have a unittest and a patch, I'll make a merge request.