main.cpp
#include <QApplication>
#include "qmlapplicationviewer.h"

#include <QGLWidget>

Q_DECL_EXPORT int main(int argc, char *argv[])
{
    QScopedPointer<QApplication> app(createApplication(argc, argv));

    QmlApplicationViewer viewer;
    viewer.setOrientation(QmlApplicationViewer::ScreenOrientationAuto);
    viewer.setMainQmlFile(QLatin1String("qml/test/main.qml"));

    //viewer.setViewport(new QGLWidget());

    viewer.setAttribute(Qt::WA_TranslucentBackground);
    viewer.setStyleSheet("background:transparent;");
    viewer.setWindowFlags(Qt::FramelessWindowHint);
    viewer.setRenderHints(QPainter::HighQualityAntialiasing | QPainter::SmoothPixmapTransform | QPainter::Antialiasing);

    viewer.showExpanded();

    return app->exec();
}

main.qml

import QtQuick 1.1
Rectangle {
    width: 400
    height: 400
    color: "transparent"

    Rectangle {
        width: 60
        height: 60
        anchors.centerIn: parent
        color: "indigo"
    }

    Rectangle {
        width: 60
        height: 60
        color: "indigo"
    }

    Rectangle {
        width: 60
        height: 60
        anchors.right: parent.right
        color: "indigo"
    }

    Rectangle {
        width: 60
        height: 60
        anchors { bottom: parent.bottom; }
        color: "indigo"
    }

    Rectangle {
        width: 60
        height: 60
        anchors { bottom: parent.bottom; right: parent.right }
        color: "indigo"
    }
}