[css-images] Add values "cover-width" and "cover-height" to object-fit

Hi,

I think this two additional values for the object-fit property would be
interesting:

- "cover-width" : keep aspect ratio and always fill the box width (but not
necessarily the box height).
- "cover-height" : keep aspect ratio and always fill the box height (but
not necessarily the box width).


-- Andrés