[css-background] Add the *-x/y longhands to appropriate properties?

Could we go ahead and add the *-x/y longhands to the appropriate
background-* properties?  I know that background-position-x/y are
already supported by several impls, and background-repeat-x/y are
supported by WK/Blink at least.

At minimum it's those two properties, though others could be expanded
out as well if necessary.

As far as I know, our current plans for handling logical-direction
background stuff are compatible with still adding physical-direction
longhands, since we'll likely do it by adding logical-direction
longhands too (background-position-inline/block, etc), plus some way
of deciding which set of positions to pay attention to.

~TJ