Use v_edgeDistance.y to differentiate between PLS strokes and fills

Now that we have two coverage components per pixel, it's cleaner to use the second, which is otherwise unused for fills.

Diffs=
4587a3d1d Use v_edgeDistance.y to differentiate between PLS strokes and fills (#5582)

Co-authored-by: Chris Dalton <99840794+csmartdalton@users.noreply.github.com>