addToRenderTargetsStack(plane) {
        // find all planes that are rendered onto a render target
        const renderTargetsPlanes = this.renderer.planes.filter(el => el.type !== "PingPongPlane" && el.target && el.uuid !== plane.uuid);

        // is there any plane that is already rendered onto that plane's render target?
        let siblingPlaneIndex = -1;
        if(plane.target._depth) {
            // findLastIndex
            for (let i = renderTargetsPlanes.length - 1; i >= 0; i--) {
                if (renderTargetsPlanes[i].target.uuid === plane.target.uuid) {
                    siblingPlaneIndex = i + 1;
                    break;
                }
            }
        }
        else {
            // findIndex
            siblingPlaneIndex = renderTargetsPlanes.findIndex(el => el.target.uuid === plane.target.uuid);
        }

        // if findIndex returned -1, just push the plane
        siblingPlaneIndex = Math.max(0, siblingPlaneIndex);
        renderTargetsPlanes.splice(siblingPlaneIndex, 0, plane);

        // sort by index (order of addition) then render order, depending on whether the render target handle depth or not
        if(plane.target._depth) {
            renderTargetsPlanes.sort((a, b) => a.index - b.index);
            renderTargetsPlanes.sort((a, b) => b.renderOrder - a.renderOrder);
        }
        else {
            renderTargetsPlanes.sort((a, b) => b.index - a.index);
            renderTargetsPlanes.sort((a, b) => a.renderOrder - b.renderOrder);
        }

        // sort by render targets order
        renderTargetsPlanes.sort((a, b) => a.target.index - b.target.index);

        this.stacks.renderTargets = renderTargetsPlanes;
    }
Add a plane to our renderTargets stack

     params:
     @plane (Plane object): plane to add to our stack
*