Clean up resources properly on scene manager destruction

QQuick3DSceneManager destructor deletes items in resourceCleanupQueue, but it doesn't properly clean up all the usage for these objects. For example, the objects in this queue can be used in QSSGBufferManager as keys in customMeshMap. If these keys are not properly cleaned, you can end up in situation where a new QSSGRenderGeometry object is created in the same memory location where an old deleted QSSGRenderGeometry object that is used for cache key was. This results in wrong geometry used for rendering. This can happen in e.g. QDS (see QDS-8516).