[css-backgrounds-4] background value definition should be redefined

[cdoublev]

background-clip is redefined with <bg-clip># in CSS Backgrounds 4 but the value definition of background only exists in CSS Backgrounds 3 and does not include <bg-clip>.

Instead of replacing (the second) <visual-box> with <bg-clip>, background (in CSS Backgrounds 3) could be defined with property value ranges to represent all its longhands. This might help clarify the position of background-origin and background-clip, which are currently both defined with <visual-box>. Besides, <bg-image>, <bg-position>, <bg-size>, do not seem useful.

[fantasai]

background will get ported to L4 eventually, it's just a diff spec right now because we're still doing major changes / clean-up (and also css-backgrounds-3 needs to get ported to modern Bikeshed markup so we can keep them in sync...)