Fix disjoint test for polygons with holes

As recommended by @awulkiew, using `point_on_border` to check whether two polygons which don't have intersecting linework are disjoint. The previous PR didn't cover all the possible cases, as @awulkiew pointed out in the comments for #156, so that case is now included as an additional test.

It is not necessary to specify the tempate-types in the call to point_on_border