NavMap filters copy the whole map and recompute every layer each cycle #123

Each cycle, every NavMap filter copies the NavMap out of NavState and writes it back, so the map is copied twice per filter. Since NavMap copies now copy their layers, this costs more than before.

• ObstacleFilter clears its layer and rebuilds it from the static map.
• InflationFilter recomputes the whole distance field. That field is also approximate.
• The maps manager publishes every filter layer in full, whether anyone listens or not.

NavMap now has what is needed to do only the work that changes: layer change tracking, an exact incremental DistanceField and sparse layer updates (EasyNavigation/NavMap#35).