Fix clipping bug for complex polygons.

Intersection points are sorted along the clipping region edge relative to a particular point.  In the case of antimeridian clipping, this point is the South pole.  Previously, the first intersection was assumed to be an entering intersection, but this is not always the case.

The fix is to see whether the clip region start point (the point relative to which sorting occurs) is inside or outside the polygon being drawn.  If it is inside, then the first intersection point must be outside, and so forth.

The same applies to d3.geo.clipExtent.  Provision was already made for this, but this has now been optimised to use a single point instead of picking one of the four corners.

Another optimisation was to reuse this clip region start point to determine whether to interpolate all the way around the clip region edge.  Previously, this was done by testing an arbitrary point in the clip region.

The above fix seemed to have broken one of the tests, and this has been fixed by modifying the point-in-polygon routine slightly to handle points that might lie exactly on the polygon edge.
    
Finally, I noticed a regression with the recent clipExtent fix, where a polygon incorrectly being marked as “clean” (no intersections) on a per-ring instead of per-polygon basis.