Spurious edge introduced when clipping open path

Hi Angus,

I'm not sure if there is interest in doing maintenance on ClipperLib, I can't help but notice there seem to be a number of unresolved geometry processing bugs. If there is interest though, here is an issue I encountered:

ClipperLib::Path a_ = {
        {1, 1},
        {2, 1},
        {2, 0},
        {4, 4},
};
ClipperLib::Path b_ = {
        {3, 0},
        {3, 3},
        {0, 3},
        {0, 0}
};
ClipperLib::Clipper c;
c.AddPath(a_, ClipperLib::ptSubject, false);
c.AddPath(b_, ClipperLib::ptClip, true);
ClipperLib::PolyTree solution;
c.Execute(ClipperLib::ctIntersection, solution, ClipperLib::pftNonZero, ClipperLib::pftNonZero);
ClipperLib::Paths solution_;
ClipperLib::PolyTreeToPaths(solution, solution_);
for (auto &e : solution_[0]) {
    auto s1 = std::to_string(e.X);
    auto s2 = std::to_string(e.Y);
    printf("%s, %s\n", s1.c_str(), s2.c_str());
}

This prints an unexpected additional edge between 1,1 and 2,0. I couldn't tell if it was related to the skip edge or not, I found that the 1,1 <-> 2,1 edge would still be reversed even if it wasn't adjacent to the skip edge. I tried to fix it, but was eventually stuck while trying to undertand the reason ProcessBounds was reversing the edge. (Maybe it shouldn't be carrying the path direction across the skip edge? That sounds reasonable, but I'm just grasping here.)

And let it not go unsaid: Thank you for ClipperLib, it has been amazingly useful to me.

Have a nice day!
-nmr