[CSS3 Background] Background shorthand

In the current definition of the 'background' shorthand, the 'background-image'
value is required (but it can be 'none') for all but the final layouer. I think
this unnecessarily complicates the syntax, and therefore I suggest we make it
optional. That way the only difference between the final layer and all other
layers will be the possible presence of a background color.

~fantasai