Making sure drawBack draws a closed outline for faces that are in the background

This is the cleaned-up version of  https://github.com/Khan/khan-exercises/pull/84085
(i.e. doesn't touch  `sketch.drawLines` )

BTW, the version which is on graphie-to-png,
http://graphie-to-png.khanacademy.org/static/js/khan-exercises/utils/graphie-3d.js
still has the bad code before the revert.