Fix #1453 - spurious interpolate when clean.

It looks to me like the d3_geo_clip polygonEnd check is assuming that if there are no clipped segments that the input polygon is outside (and possibly surrounding) the viewport. However, the clipped segments can also be empty when the polygon is completely inside the viewport, in which case the listener was already sent the input polygon (clean & 1). This check prevents additional interpolation when there was no intersection.