…on bridges

- Do not hide secondary road bridges on z12
- Do not hide tertiary, residential, unclassified, and road bridges on z13

Conflicts:

	roads.mss

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 gravitystorm#269 on Github and 3883 on trac.
* Restore bridges of bridleway, footway, cycleway, path, lightrail on
  z13.

- Make background of path in tunnels wider, so that it does not
  disappear below the path itself
- Line-opacity of background of path should only apply to regular
  roads, not to tunnels