[regression] ShaderEffectSource should be able to duplicate content across Windows

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    engine.load(QUrl(QStringLiteral("qrc:/Combined.qml")));

    return app.exec();
}

import QtQuick 2.3
import QtQuick.Controls 1.2
import QtQuick.Window 2.2

Item {
    width: 300
    height: 300

    Window {
        x: 500
        y: 100
        width: 640
        height: 480
        visible: true

        Item {
            id: foo
            width: 100; height: 100
            Rectangle { id: rect; x: 5; y: 5; width: 60; height: 60; color: "red"
                NumberAnimation { target: rect; property: "rotation"; duration: 1000;
                    from: 0
                    to: 360
                    easing.type: Easing.Linear
                    loops: Animation.Infinite
                    running: true
                }
            }
            Rectangle { x: 20; y: 20; width: 60; height: 60; color: "orange" }
            Rectangle { id: yellowRect; x: 35; y: 35; width: 60; height: 60; color:mouse.pressed ? "blue" : "yellow"
                MouseArea {
                    id: mouse
                    anchors.fill: parent
                    drag {
                        target: parent
                        axis: Drag.XAndYAxis
                    }
                }
            }
        }

        // this shader effect source hides the foo item
        ShaderEffectSource {
            width: 100; height: 100
            sourceItem: foo
            smooth: true
            antialiasing: true
            hideSource: true
        }

        // another shader effect source in the same window is OK
        ShaderEffectSource {
            x: 200
            width: 100; height: 100
            sourceItem: foo
            smooth: true
            antialiasing: true
        }
    }

    // another window
    Window {
        x: 100
        y: 100
        width: 640
        height: 480
        visible: true

        // this shader effect source causes the windowRef count to exceed 1 and cause the warning:
        // Qt5.3: QQuickItem: Cannot use same item on different windows at the same time.
        // Qt5.4: ShaderEffectSource: sourceItem and ShaderEffectSource must both be children of the same window.
        ShaderEffectSource {
            width: 100; height: 100
            sourceItem: foo
            smooth: true
            antialiasing: true
        }
    }
}