Refactor pedestrian and living_street

* Road widths pedestrian and living_street changed slightly to make them more
  consistent, both internally and with other roads
* Change colour of casing pedestrian and living_street (resolves #708)
* Solve various bugs/inconsistencies:
  - Render pedestrian bridges from z14 instead of z13
  - Add round line-join on living_street bridges
  - Render living_street fill in tunnels only from z13
  - Remove round line-cap from pedestrian tunnels (resolves #900)
* Road widths pedestrian and living_street are now defined with variables
* In general make code more consistent with other road types