svg overlays instead of builtin leaflet drawing

[Gjum]

maybe use d3 for that:

• https://github.com/teralytics/Leaflet.D3SvgOverlay
• or https://github.com/christiankaiser/Leaflet.D3SvgOverlay
• https://bost.ocks.org/mike/leaflet/

need to make sure it still renders fine on all devices, leaflet has a canvas fallback

allows us to draw smooth svg curves https://www.w3.org/TR/SVG/paths.html#PathDataCurveCommands

and to use TexturesJS https://riccardoscalco.github.io/textures/

[Gjum]

this also allows us to use automatic label placement