Problem with BrightnessContrast QML Type

[Czarek]

Hello,
I would like to implement something which lets user of my program to change brightness and contrast. I tried BrightnessContrast QML Type, but it "covers" places with mouseAreas making them useless. My tree:

• Item
  • MainScreen (rect)
  • RowLayout
    • Buttons
      • mouseAreas
  • BrightnessContrast

When I gave RowLayout bigger z coordinate than BrightnessContrast, buttons were visible but mouseAreas were unable. I tried to put mouseAreas outside RowLayout - I made mouseAreas and BrightnessContrast siblings. Then I tried to manipulate with z coordinate, but mouseAreas are still useless. Could you tell me, why mouseAreas are unable to use? Maybe you have better idea how can I reach what I scheduled? Please help.

[Czarek]

import QtQuick 2.4
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.1
import QtQuick.Extras 1.4
import QtQuick.Window 2.2
import QtQuick.Controls.Styles 1.4
import QtGraphicalEffects 1.0

Item {
    width: 640
    height: 480
    id: item

    property alias text1: text1
    property alias mouseArea1: mouseArea1
    property alias mouseArea2: mouseArea2

    Rectangle {
        id: okno_glowne
        color: "#bb8a63"
        anchors.fill: parent

        Text {

            id: text1
            x: 499
            y: 193
            width: 65
            height: 42
            font.pixelSize: 33
            text: "pampam"
        }
    }

    states: [
        State {
            name: "drugi_ekran"
            PropertyChanges { target: okno_glowne; visible: false  }

            PropertyChanges {
                target: drugie_okno
                x: 163
                y: 75
                opacity: 1
            }
        }
    ]

    Rectangle {
        id: drugie_okno
        color: "#bb8a63"
        anchors.fill: parent
        opacity: 0
    }

    RowLayout {
        id: rowLayout1
        x: 170
        y: 322
        z:3
        width: 300
        height: 150

        Button {
            id: button1
            text: qsTr("Button")
            anchors.rightMargin: 150
            anchors.fill: parent
            width: 150
            height: 150
            z:3

            MouseArea {
                id: mouseArea1
                x: 0
                y: 0
                z:3
                anchors.fill: parent

                width: 150
                height: 150
                enabled: true
                visible: true

            }

        }

        Button {
            id: button2
            text: qsTr("Button")
            anchors.leftMargin: 150
            anchors.fill: parent
            width: 150
            height: 150
            z:3

            MouseArea {
                id: mouseArea2
                x: 0
                y: 0
                z:3

                anchors.fill: parent
                width: 150
                height: 150
                enabled: true
                visible: true
            }            
        }        
    }

    BrightnessContrast {
        id: asd        

        anchors.fill: okno_glowne
        source: okno_glowne
        brightness: -0.6
        contrast: -0.6
        z:0
    }
}

Do You see any mistakes in the code?