[Tilesathome] New waterway rendering

[Patrick Kilian]

Hi all,

could somebody please review the patch at
http://trac.openstreetmap.org/attachment/ticket/1294/waterway-v3.patch

It is a complete overhaul of the rendering of the waterways. Waterways
with "natural" borders (river, stream) have no gray casing, waterways in
  artificial borders (drain, canal) have a gray casing (think concrete)
in levels z15 - z17.
This also fixes the bug that a way tagged as river between two ways
tagged as riverbank (very common for wide rivers) appears as thin blue
way with gray casing on top of the blue area defined by the riverbanks.
(no trac ticket for this issue.)

Waterways with k="tunnel" v="yes" are shown as dash blue on solid gray.
This fixes trac ticket #1294.

I also adjusted the widths with the following constraints:
* At z17 the waterways are shown at the correct width of 3 / 10 meters.
* At z12 the waterways retain the old widths of 4 / 10 svg pixels.
* The changes between the levels are a constant multiplicative factor to
create smooth transitions between the level.
* The changes between the levels are small enough that the same way
never has more pixel in the png after zooming out. Thus fixes trac
ticket 1263


If sufficient interest exists I could also adjust the z10 and z11
stylesheets to match.

Patrick "Petschge" Kilian