Bounding rectangle of rotated Item

MattB.au

How do you access the bounding rectangle of a rotated QML Item? For example in the code below how would I make the parent 'boundingRect' match the extents of the rotated child.

@
import QtQuick 2.1
import QtQuick.Controls 1.0

ApplicationWindow {
id: mainWindow
width: 300
height: 300

Rectangle {

    id: boundingRect
    x: 50
    y: 50
    color: 'yellow'

    width: childrenRect.width
    height: childrenRect.height

    Rectangle {
        id: rotated
        width: 75
        height: 100
        border.width: 1
        border.color: 'blue'
        rotation: 45
    }
}

}
@

Started to look at a solution using a width/height based on mapFromItem() but I don't think that's going to work as a general solution when I have multiple children which may be rotated and I want the bounding box of all of them.

Thanks