UI to take a snapshot of a running program.

Users in most browsers can right-click on pictures and "Save Image As..." to save an image from their code. However, there are weaknesses:

1. Resolution is determined by the browser window.
2. Hovering over a `drawingOf` program makes debug controls appear, and it's not obvious how to get rid of them.

Instead, we should extend the button that currently records video to be able to grab a static image as well. Ideally, you'd get a choice of image size, though that's not necessary.