Add indoor layer #2
Description
One of only top-level tags that Sourdough doesn't yet offer a layer for is indoor=*, which is used to map interior spaces (rooms, hallways, etc) in buildings.
The reason Sourdough does not yet include indoor features is that it isn't practical to include at z15, which is the maximum zoom currently supported by Planetiler. Indoor mapping is sparse in OSM, but where it does exist, it has the potential to add a lot of data to the tiles. For example, when I tried including indoor=* features in Sourdough, the tile 15/6826/12435 (which includes most of Auraria Campus in Denver, CO) ended up being 589 KB, with indoor features accounting for 80% of the total. When tile sizes exceed a couple hundred kilobytes, they take longer to download, and they also tend to cause long GC pauses in MapLibre GL JS when they are disposed of.
I'd like to include indoor features in Sourdough, but only plan to do so if Planetiler adds support for generating z16 tiles. The maps I tested (including indoorequal.org, Apple Maps, and Google Maps) don't begin showing indoor details until z16 or z17, so I think this should be fine for most use cases. And it should help keep individual tile sizes manageable (though adding one more zoom level will make the tileset as a whole take ~4x as much space, so some users might choose to generate tiles only up to z14 or z15 if they don't need these additional details for their maps).