Optimize Polygon subdivision

Increases performance and reduces memory allocations. The times in the table below are in ms.

| Polygons | master | This branch |
| ------------ | --------- | --------------- |
| US states | 59.7 | 43.4 |
| US counties | 930.4 | 257.4 |
| Countries | 10747.2 | 5620.7 |