Properly load lazy HTML elements before taking screenshots.

[dalin-]

In the issue queue there's several suggestions about how to lazy load images. However they are all a bit lacking. They still require adding delay or postInteractionWait to wait for the images to load. To do it right we need a promise.

Here's a draft PR that does it marginally better. But images aren't the only thing that can be lazy. I haven't tested this code on lazy iframes (and the variables should be renamed to indicate that).

I'm also not a JS engineer (is it obvious). So this might be improved even more.