image-filters:blur does not work as expected

[springmeyer]

Its quite nice looking, but does not appear blurred (agg-stack-blur does work):

done with:

Map {
  background-color: steelblue;
}

#countries {
  polygon-fill: green;
  image-filters:blur;
}

[springmeyer]

/cc @artemp - this is still present as an issue after alpha fixes. What do you think about just aliasing blur -> agg-stack-blur. Is there really any reason to keep around the existing blur?