Clean up waterways

* Merge definitions of canal and river (canals are now as wide as rivers
  instead or wider on some and narrower on other zoomlevels).
* Merge definitions of stream, drain, and ditch.
* Render river/canal in tunnel like stream/drain/ditch in tunnel.
* Render river/canal in tunnel in tunnel-style on z12 and z13.
* Render tunnel=culvert like tunnel=yes.

This solves the following issues:
* Fixes 3590 (trac): Canals wider than rivers at z12, disappear at z11
* Fixes 4227 (trac): canals as tunnels render differently at low zooms
* Fixes 4507 (trac): Waterways in a tunnel may be confused for roads
* Fixes 4656 (trac): waterway=canal too wide compared with highways at z14
* Fixes #198: Add rendering of tunnel=culvert for waterway
* Fixes #222: Inconsistent water-lines-casing tunnel check
* Fixes #282: "waterway=stream" with "tunnel=yes" nearly invisible

With thanks to @yarl for the work on #198.