Stroking fixes/improvements

1. **Help, I can’t decide how bad/good my commit message is again.**

    I suppose it would also be nice if someone measured how much of a slowdown this is, but I don’t think the issue can be fixed in any other way.

    Test cases: anything that has self-intersections. Simple artificial example: `{\p1}m 0 0 l 200 200 200 0 0 200`. Don’t forget to give it a non-zero border.

2. Do we want version macro checks?
    * We could raise the minimum version requirement.
    * Do we want a runtime version check? Older FreeType is ABI-compatible but doesn’t have this function.
    * We could put this change off until such a time when we’re happy to raise the minimum version requirement. I didn’t actually notice any difference in the output, although I only did one quick test.