regression: subpixel-aa isn't used on fast-shadows

[Gankra]

Was fixed in #1483 and there's a test for it but it's commented out saying "needs issue #1483"

[Gankra]

(at least) the following gecko tests fail on windows because of this:

== text-shadow/blur-opacity.html text-shadow/blur-opacity-ref.html
== text-shadow/basic.html text-shadow/basic-ref.html
== text-shadow/basic-negcoord.html text-shadow/basic-negcoord-ref.html
== text-shadow/basic-opacity.html text-shadow/basic-opacity-ref.html
== text-shadow/color-inherit.html text-shadow/color-inherit-ref.html
== text-shadow/multiple-noblur.html text-shadow/multiple-noblur-ref.html
== text-shadow/quirks-decor-noblur.html text-shadow/quirks-decor-noblur-ref.html
== text-shadow/standards-decor-noblur.html text-shadow/standards-decor-noblur-ref.html
== text-shadow/overflow-decoration.html text-shadow/overflow-decoration-ref.html

[glennw]

This was fixed - somewhat related to #2384.