[css-images-4] Resolution negotiation should be linked to srcset spec

[gi11es]

At the moment the handwavy definitions are independent, which means that the UA might implement different logic there, making image-set incompatible with the newly introduced imagesrcset preloading capability (w3c/preload#120).

If the definition of the resolution negotiation was shared between both specs (whatwg html and css images), it would enforce that browsers use the same logic for these related features, making imagesrcset preloading a feature that can be combined with image-set.

At the moment the implementation of both on Chrome 73 is incompatible at certain zoom/density levels, making the wrong size preloaded for an identical definition of source sets between a link with imagesrcset and an element with image-set.