Improve labeling of high-tortuosity LineStrings with LineLabelSymbolizer

[bdon]

https://en.wikipedia.org/wiki/Tortuosity

Create a sample dataset from ne_10m_rivers_lake_centerlines

./tippecanoe -o rivers ne_10m_rivers_lake_centerlines.fgb
pmtiles-convert rivers.mbtiles rivers.pmtiles

should show at least some labels