Scene3D shutdown doesn't work properly in Loader.

It seems that there are some bugs in scene shutdown code, which is prone to memory-leaks and crashes.

Steps to reproduce:

Open scene3d-loader example and add follow snippet to the main.qml:

Timer {
        interval: 1000
        running: true
        repeat: true
        onTriggered: loader.source == "qrc:/Scene.qml" ? loader.source = "Scene2.qml" : loader.source = "Scene.qml"
    }

The bug(s) reproduceable in different conditions with release and debug builds. 100% crash in Valgrind.
The memory usage grows by about 70 mb per switch.