Redact content as text and background image

[stefanw]

When the content is available as invisible overlay text and in the background image, both must be redacted, optimally in the same step.

Redacting the image first, makes the text harder to find. Redacting the text first, makes it look like everything is redacted.

Possible solution: check if there is an image which has the same size as the page, then it is the background (should be the first element). Then we know if there's a background that needs to get redacted as well.