Waiting for webgl to finish rendering using fenceSync

sync.js

    // assembled from some stackoverflow answers
    
    const start = performance.now();

    // Draw the fullscreen quad
    twgl.drawBufferInfo(gl, quad_bufferInfo);

    let sync = gl.fenceSync(gl.SYNC_GPU_COMMANDS_COMPLETE, 0);

    checkSync = () => {
        const status = gl.clientWaitSync(sync, 0, 0);
        switch (status) {
            case gl.TIMEOUT_EXPIRED:
                return setTimeout(checkSync);
            case gl.WAIT_FAILED:
                throw new Error('should never get here');
            default:
                gl.deleteSync(sync);

                const duration = (performance.now() - start);
                console.log(`finished rendering in ${duration.toFixed(1)}ms`);

                // trigger preview capture now that GL has finished rendering
                triggerPreview();
        }
    }
    setTimeout(checkSync);