Fix a few topology.filter bugs and reverse holes.

* Counterclockwise ring areas were being incorrectly calculated (the
  area A is already negative so the positive area is π + A).
* The MultiPolygon filter should *exclude* polygons with an area smaller
  than the minimum.
Better tests for topojson.object.

Also fix a bug where LineStrings could have only one coordinate rather than the
required two, and another bug where null geometry objects lost their id and
properties.