An empty Scene3D causes a high CPU load

This bug report was cloned from AUTOSUITE-798. Further investigations has shown that a similar problem occurs on a pure desktop build with not Qt Auto components involved. See comments on AUTOSUITE-798. Below is an extract:

**

desktop:

OS: Ubuntu 16.04 xenial
Kernel: x86_64 Linux 4.15.0-45-generic
Resolution: 5360x1440
DE: Unity 7.4.5
WM: Compiz
WM Theme: Ambiance
Icon Theme: ubuntu-mono-dark
Font: Ubuntu 11
CPU: Intel Core i7-7700 CPU @ 4.2GHz
GPU: Mesa DRI Intel(R) HD Graphics 630 (Kaby Lake GT2)
RAM: 11010MiB / 15905MiB

 echo $XDG_SESSION_TYPE -> X11

build with self-built Qt version 5.12.2 as:

/opt/checkout/qtbase/configure -top-level -opensource -confirm-license -nomake examples -nomake tests -opengl es2 -prefix /opt/qt/auto/5.12.2
rev: 83d5226604365756adae991c4787edaf6d6b5c6e

1 window:

import QtQuick 2.9
import QtQuick.Window 2.2

import QtQuick.Scene3D 2.0

Window {
    visible: true
    width: 1920
    height: 1080

    Scene3D {
        id: scene3d
        anchors.fill: parent
    }
}

Self-built Qt: ~9%cpu

2 windows:

import QtQuick 2.9
import QtQuick.Window 2.2

import QtQuick.Scene3D 2.0
QtObject {
    property Window w1: Window {
        visible: true
        width: 1920
        height: 1080

        Scene3D {
            id: scene3d
            anchors.fill: parent
        }
    }

    property Window w2: Window {
        visible: true
        width: 1920
        height: 1080

        Scene3D {
            id: scene3d2
            anchors.fill: parent
        }
    }
}

~ 13-14%