Merge layers of bridges (for each osm-layer)

For each osm-layer, we merge the directions layer of bridges into the general layer of bridges.
This is done in preparation for removing the duplication of bridge layers for each osm-layer.

This does not affect rendering.
Merge branch 'roadsv3.0' into roads-bridges
Add missing 'directions' classes
Merge branch 'roadsv5a.0' of https://github.com/math1985/openstreetmap-carto into math1985-roadsv5a.0