Label density can be optimized at z8

[wipfli]

In particular at z8 many labels in urban areas cannot be rendered due to collisions. Below is an example of New York, rendered with maplibre map.showSymbolCollisions = true. Maybe we can optimize tile size and bring fewer labels to the renderer which means fewer collision computations. Could make sense to have a symbol pruning step in Planetiler (cc @msbarry).