Collapse definitions for normal roads, tunnels and bridges

This collapses, per road type, definitions that are the same for normal
roads, tunnels and/or bridges.
In addition, it gives some instances clearer names.
This makes it more easy to maintain the code, and to detect inconsistent
definitions.

Changes in rendering:
* Render minor service roads on bridges and tunnels smaller and on higher
  zoomlevels only, just like minor service roads that are not on a
  bridge or tunnel. This solves #269 on Github and 3883 on trac.
* Restore bridges of bridleway, footway, cycleway, path, lightrail on
  z13.

project.mml

@@ -833,7 +833,7 @@
       ],
       "Datasource": {
         "type": "postgis",
-        "table": "      (select way,coalesce(('highway_' || (case when highway is not null then highway else null end)), ('railway_' || (case when railway in ('rail','tram','light_rail','funicular','narrow_gauge') then railway else null end))) as feature,tunnel\n       from planet_osm_roads\n       where highway is not null\n          or (railway is not null and railway!='preserved' and (service is null or service not in ('spur','siding','yard')))\n       order by z_order\n      ) as roads_low_zoom",
+        "table": "      (select way,coalesce(('highway_' || (case when highway is not null then highway else null end)), ('railway_' || (case when railway in ('spur','siding') or (railway='rail' and service in ('spur','siding','yard'))  then 'INT-spur-siding-yard' when railway in ('rail','tram','light_rail','funicular','narrow_gauge') then railway else null end))) as feature,tunnel\n       from planet_osm_roads\n       where highway is not null\n          or (railway is not null and railway!='preserved' and (service is null or service not in ('spur','siding','yard')))\n       order by z_order\n      ) as roads_low_zoom",
         "extent": "-20037508,-19929239,20037508,19929239",
         "key_field": "",
         "geometry_field": "way",