Bug 1064110 - Add support for stroke scale modes

Can you expand on the difference? I wouldn't mind a fairly long comment explaining the quirks. Eventually we'll have to revisit this, and without such a comment, we'll have to start from the beginning. To describe the basics, you could crib from my description in [bug 1064110](https://bugzilla.mozilla.org/show_bug.cgi?id=1064110).