Provide split and unsplit road layers

[opatut]

Talking about the directional split here -- there should be a layer obs_roads and obs_roads_split, only one of which is split by direction like now. The logic should be the same, except that overtaking_events on the unsplit layer are not filtered by direction.

This will make it easier to build nice-looking styles like a event density or road usage style, especially on low zoom levels (10.00-12.99).