[css4-background] Drop restriction for color on last background layer only

This always seemed an unnecessary restriction to me as an author, and I 
recall needing a background color in other layers multiple times. The 
use cases I've stumbled on,  usually fall in one or both of the 
following categories:

a) A semi transparent color with background-image(s) showing through 
below it
b) A color clipped with a  background-clip value of padding-box or 
content-box with background-image(s) clipped with border-box or 
padding-box showing through.

I've had a discussion today with Tab and he encouraged me to raise this 
on the list, as:

a) it hasn't been discussed before (I have to admit I found this kinda 
surprising)
b) There are no known implementation issues with shifting this restriction
c) This restriction was added to the spec before background-clip was, so 
the use cases were more limited

Yes, both use cases can be solved with the image(color) and cross-fade() 
syntax, but allowing a background-color in other background layers is 
simpler and more straightforward.

-- 
Lea Verou (http://leaverou.me | @LeaVerou)