add missing call to drawScene

Author: greggman

webgl/webgl-tips-screenshot-good.html

@@ -216,6 +216,7 @@
 
   const elem = document.querySelector('#screenshot');
   elem.addEventListener('click', () => {
+    drawScene();
     canvas.toBlob((blob) => {
       saveBlob(blob, `screencapture-${canvas.width}x${canvas.height}.png`);
     });