Fix filtering of small polygons with holes.

(Or, the unusual case of Bora Bora.) When filtering polygons, the old
implementation could filter the exterior ring of a polygon but keep its interior
rings, thus promoting the first interior ring to the exterior!
Fix filtering for holes.

We should use the inverted area to determine whether to filter holes, since
we’re trying to measure the area of the hole, not that area around the hole. But
note that it’s still faster to check the exterior ring first before checking any
interior rings.
More filter tests.
Merge branch 'fix-filter'