Render center dots for highways that have no sidewalks but are foot=yes.

The intended meaning of this is assumed to be "pedestrians are allowed
to walk in the road", which is illustrated as a dotted line down the
middle of the road area, rather than at the edges as we'd do for
sidewalks.
Show steps as a line of wider dots.
Make all roads white, rather than just the ones that are inaccessible to peds.
Ignore underground or underwater buildings.
Update our OSM extract.