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.

Start at the existing button that records video and trace how it interacts with a running drawingOf program in the browser. Check how debug controls appear during hover and how the current rendering is exposed for saving; done means the button can capture a static image without those controls, with image-size selection if practical.

Vom Indexierungsmodell aus dem Issue-Text verfasst.