getWebGLBoundingRect() {
        if(!this._matrices.modelViewProjection) {
            return this._boundingRect.document;
        }
        else if(!this._boundingRect.worldToDocument || this.alwaysDraw) {
            this._computeWebGLBoundingRect();
        }

        return this._boundingRect.worldToDocument;
    }
Returns our plane WebGL bounding rect relative to document

     returns :
     @boundingRectangle (obj): an object containing our plane WebGL element bounding rectangle (width, height, top, bottom, right and left properties)
*