offscreen-images: allow eager loading within three viewports

[brendankenny]

offscreen-images suggests lazy loading any images outside of the initial viewport (plus a little leeway on all sides).

After discussion in ampproject/amphtml#25682 and in the latest eng meeting, we've decided to bump that up to three viewports. It's still a little arbitrary, but the impact is minimal, it lines up with the leniency of other lazy loading approaches (like the very large buffer in native lazy loading), and it's arguably a better user experience.

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whatwg discussion on this issue: whatwg/html#5408

[addyosmani]

To add one small note from the sync: one of the nice things about the three viewports solution is that it doesn't require us to introduce special-casing for every lazy-loading solution (which could get a bit out of hand over time...). Instead, this should be lenient enough to cater for a number of approaches as Brendan mentioned.