The screenshot track should make it obvious when something has changed on screen

As currently displayed, the screenshot track makes it easy to see when a big change (eg. drawing the main content of a web page) is painted on the screen, but smaller changes are hard to spot.

One possible way to address this could be to stop repeating identical screenshots. Instead we could display one screenshot at the beginning of the track, and then only display a screenshot in the track when something has changed. Hovering the track at any point could still show a larger version of the screenshot that was current at that point in time, even if the hovered point of the track doesn't show an image.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-120)