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.