The opacity property should not be applied to children of an element but apply evenly to the element and the children

[tronical]

The following test case is currently rendered incorrectly (different than the way for example opacity works in CSS):

export Demo := Window {
    preferred-width: 800px;
    preferred-height: 600px;

    Rectangle {
        opacity: 0.5;
        width: 100px;
        height: 100px;

        background: red;

        Rectangle {
            background: blue;
            width: 50px;
            height: 50px;
            x: 25px;
            y: 25px;
        }
    }
}

Current (wrong) output (with the GL backend):

However the red and the blue rectangle should be rendered without opacity and then blended against the background with the specified opacity: