Fix PolygonRef::inside

The function PolygonRef::inside fails when the tested point is at the same Y value than an edge of the polygon, like in the following example.

![inside_fix](https://cloud.githubusercontent.com/assets/6553898/3634591/8abacf2a-0f3a-11e4-8560-742c0fdecc49.png)


The red point is considered in the yellow triangle, this is obviously wrong.
It is possible to test the issue with the following test case:
<pre>
void inside_testcase()
{
    Point p;
    p.X = 0;
    p.Y = 1;
    Polygon poly;
    {
        Point pt;
        pt.X = 1;
        pt.Y = 0;
        poly.add(pt);
    }
    {
        Point pt;
        pt.X = 2;
        pt.Y = 1;
        poly.add(pt);
    }
    {
        Point pt;
        pt.X = 3;
        pt.Y = 0;
        poly.add(pt);
    }
    if (poly.inside(p))
        printf("Point in triangle (this is wrong!)\n");
    else
        printf("Point out of triangle\n");
}
</pre>
My fix consists in using the function <i>PointInPolygon</i> provided by Clipper.