Sanitizing out screenshots should keep CompositorScreenshot markers without the image

When we share a profile without screenshots it is typically because there was personal information on screen. It would be nice to still share the screenshot markers, without the images, so that when looking at the profile, it remains easily possible to see which window changed on screen, and what the size of the windows were.

Even better than removing the images might be to blur them entirely, or make them a solid color, that changes whenever 2 screenshots are different. Showing on the image which area of the screenshot differed would be even better.

┆Issue is synchronized with this [Jira Task](https://mozilla-hub.atlassian.net/browse/FP-566)