[css-backgrounds] Add 'scale-down' to background-size

[jez9999]

This is for css-backgrounds-4 (I can't seem to add labels). Link to spec section:
https://drafts.csswg.org/css-backgrounds-4/

As evidenced by various threads like this one on StackOverflow:
http://stackoverflow.com/questions/21407466/css-background-size-contain-without-the-image-to-scale-up

Quite a few people (myself included) would like the functionality that background-size:contain provides, except that when the image is smaller than the container, it should not scale up. Perhaps we could add background-size:scale-down?

[zcorpan]

'object-fit' has a scale-down value with this behavior.
https://drafts.csswg.org/css-images-3/#the-object-fit

[jez9999]

Good point; I've updated the title to fit.