Slippy tile graph inspector

Add an API to render a slippy tile layer on top of a map of the covered graph region, rendering various graph information for inspection/debugging purpose (bike safety factor, connectivity...).
Can easily be extended if needed to include more layers.
This rely on a modification to the spatial index to include all edges with geometry, not only street edges. This should not have big impact in term of performance, street edges are the majority of edges anyway.