Following on from #1686, we drop small courtyards before and after merging adjacent buildings. However, in some places (e.g: Paris, below), the building footprints are extremely complex, with a lot of internal structure; courtyards, lightwells, etc...

Many of these are superfluous at zoom 15 and add too much detail. We should consider increasing the area threshold for keeping these inner rings in the output, and tune the value to the data.