Addressed Domenic's comments"

Various nits and cleanups:

- Remove "lazyload hint" concept and refer instead to states of the attribute directly
- IDL attribute casing should be lazyLoad, not lazyload
- Be a bit more precise in the iframe processing model
- Remove domintro for reflecting attributes (both this one and the one added for img.decoding, erroneously)

Updated metadata fetch algorithm

Addressed comments

Referred to add a range header

Addressed nit

Source formatting nits

Added info about how to handle range response

Addressed some comments from domenic

Addressed remaining comments from domenic and others

Addressed more comments from Domenic

Editorial tweaks

Avoid potential race condition by saving corsState and origin

Have an explicit "el" variable

Previously it was sometimes implicit, sometimes "the img element", sometimes "the element"...

DRY up the code a bit with a "document" variable

Since a document's origin can never change, this is safe to do.

Upon re-reading, moving this sentence up seems clearer

Changed to load=lazy|eager|auto

Wait for the element to be in the active document.

Attribute renamed as 'loading' and added intersection observer algo

Addressed more comments

Images created by the NamedConstructor should be eager by default

Yank 'fetch image metadata' + small fixes

Revert 'Images created by the NamedConstructor should be eager by default'

Properly link to IntersectionObserver

Always invoke updating-the-image-data algorithm

This commit removes the option to defer updating-the-image-data
based on the loading attribute's state, and instead invokes it
unconditionally. Then, instide the updating-the-image-data algorithm,
the UA can optionally continue fetching the image normally, or wait
until the image intersects with the viewport, or the loading attribute
changes values.

Add privacy fingerprinting notes + remove superfluous link

Small fixes

Move deferral of image loading after request is made

Editorial: Attempt to fix Travis build

updating-the-image-data should not await a stable state

Rebase

* Updating-the-image-data cannot be called while in-parallel

Fix rebase mistake (loadstart event)

Refactor lazy loading conditions for images

Limit invocations of compute the intersection

Remove the <iframe> bits

Stop referring to the lazy load attribute as a hint

Introduce lazy loading mode abstraction

Remove newlines

Remove auto + lazy loading mode abstraction, explicitly delay the load event, reformat lazy loading conditions

Fix backwards logic with delaying the load event

Formatting

Easy stuff...

Remove IO dependency, note larger intersection areas than viewport

Add lazy load example with loading explanation, and dev note

Fix error from removing compute-the-intersection

Address reviews

Address nits + move dfn intersect the viewport