[css-backgrounds] Proposal: add fill keyword to background-size

[benface]

object-fit has two keywords that background-size doesn't have (other than none which is pretty much the equivalent of background-size's auto): scale-down and fill.

Issue #468 exists for adding the scale-down keyword to background-size.

I am creating this issue to suggest adding fill as well. It would be the equivalent of background-size: 100% 100%.