BorderImage does not update source property

This issue was reported against Qt Quick Controls but there seems to be a more serious regression which prevents BorderImages from repainting when the source property changes.

import QtQuick 2.1
BorderImage {
    source: mouse.pressed ? "button.png" : "button_down.png"
    MouseArea {
        id: mouse
        anchors.fill: parent
    }
}

Note the issue was originally reported against controls but can be traced back to this.