Clean-up paths and tracks, in particular on z13/z14

- Make paths and tracks less visible on z13/z14, by making them narrower and
  remove the casing at these zoomlevels. This declutters in high-density
  environments, while at the same keeping the paths and tracks visible in
  low-density environments.
- Hide private paths and tracks on z13/z14.
- Define path/track widths with variables.
- Make widths of path/track bridges and tunnels more consistent.
- Add rendering for steps in tunnels.
- Add background (glow) to steps, just like footways.

This solves the following issues:
* #211 (Poor result of rendering areas with many footways in zoom level 13)
* #620 (Tracks too dominant on z13)
* Trac 1508: don't render tracks of high tracktype at low zoom (-> wontfix)
* Trac 3788: Don't render highway=track, access=private at z13&14
* Trac 4015: Rendering of highway=path with access=no is inconsistent
* #634: private footways should not be rendered up to z13
Merge branch 'paths-z13' of https://github.com/math1985/openstreetmap-carto into math1985-paths-z13
Merge branch 'math1985-paths-z13'