drawStack(stackType) {
        for(let i = 0; i < this.stacks[stackType].length; i++) {
            const plane = this.stacks[stackType][i];
            // be sure the plane exists
            if(plane) {
                // draw the plane
                plane._startDrawing();
            }
        }
    }
Loop through one of our stack (renderTargets, opaque or transparent objects) and draw its planes
*