GeoInteface.getgeom(polygon, i) is very slow. We need a cache for calculating holes and rings only once.

[rafaqz]

Using getgeom(multipolygon) rather than getgeom(polygon, i) is recommended... but in practice the later is used a lot and is currently insanely slow if you have e.g. 100 holes in a polygon.

We could maybe add a cache of bit indices for each polygon with the exterior then all the holes. Then we can just index with it to get each polygon after the first time.