Merge layers for bridges using group-by

The 6 layers for bridges (one per osm-layer) are merged and replaced by a group-by construction.
This removes duplication in the code.

This change does not affect rendering.
Merge branch 'roadsv3.0' into roads-bridges2
Merge branch 'roads-bridges' into roads-bridges2

Conflicts:
	project.mml
Merge branch 'roadsv5a.1' of https://github.com/math1985/openstreetmap-carto into math1985-roadsv5a.1
Use 'layer in (...)' instead of repeated 'layer=...' in query.