Anti-aliasing with plane splitting

[kvark]

We have a disabled non-trivial plane splitting test:

#== cross.yaml cross-ref.yaml #TODO: investigate sub-pixel differences

It would be great to enable. I did investigate, and the reason appears to be just the lack of AA on the plane splitting rendering path.

[kvark]

See also - https://bugzilla.mozilla.org/show_bug.cgi?id=1737904