setCSSPerspective() {
        this.CSSPerspective = Math.pow( Math.pow(this.width / (2 * this.pixelRatio), 2) + Math.pow(this.height / (2 * this.pixelRatio) , 2), 0.5 ) / Math.tan((this.fov * 0.5) * Math.PI / 180);
    }
Sets a CSSPerspective property based on width, height, pixelRatio and fov
     Used to translate planes along the Z axis using pixel units as CSS would do
     Taken from: https://stackoverflow.com/questions/22421439/convert-field-of-view-value-to-css3d-perspective-value
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