Add support for rendering "natural=tree-row".

Very handy for walking navigation, and it prevents from tagging for the
renderer by mapping things as wood/forest instead for a small section between
fields or f.e. near railways.
Added missing comma, which caused Tilemill-error

Resolves the following Tilemill-error:

landcover.mss:239:32 [[feature]=power_substation] added to
[feature]=power_sub_station produces an invalid filter
Merge pull request #551 from floscher/comma

Added missing comma, which caused Tilemill-error
Order tunnels by layer

This makes sure that the group-by on layer for tunnels works correctly,
and that tunnels are correctly stacked per layer.
Render building passages as tunnels (correction)

Building passages are now correctly rendered as tunnels.
This fix makes sure that such tunnels are not being hidden by the
non-tunnel rendering, which was rendered in addition to the tunnel
rendering.

This is a correction of #444.
Implement track style changes from #136

This is a rewrite of #205

Closes #205

Fixes #446
Small corrections to track style changes

- Remove comp-op src-over
- Remove line/background attachments
- Remove named bridgecase instance
- Correct comment syntax
- Remove opacity attribute for tunnels
- Decrease line-opacity of background
Remove special layering treatment sport areas

Sport areas are now ordered just like other landcover areas, i.e. by
z_order and way_area, instead of on top of other landuse.
In addition, pitches and tracks are no longer automatically rendered
on top of stadiums, as way_area should take care of that.

This resolves #394 on Github and #3224 on trac.
Made some line-colors dependent on their polygon-fill

Objects with the following tags have now a border-color, that depends on
the polygons fill-color:

leisure=swimming_pool/playground/track/pitch
tourism=camp_site/caravan_site/picnic_site
landuse=field/retail
natural=field
power=station/generator/sub_station/substation
amenity=parking
aeroway=aerodrome

The dependency of both colors was expressed by adding:
line-color: saturate(darken(@color, x%), y%)
with 40 <= x <= 60
and 0 <= y <= 30
Fix rendering of power=substation

Add SQL change missing from bebb020d2c0e8
Added color-variables for buildings and made their line-color dependent on their polygon-fill

Affected tags:
* building=*
* aeroway=terminal
* railway=station
* amenity=place_of_worship
Rationalize road sizes

These changes make sure that more important roads are not rendered
smaller than less important roads.

- Render motorways from z12 on wider.
- Render trunk, primary, secondary, and tertiary on z13 and z14 narrower.
- Render casing on tertiary link wider.

This solves #256.
Make road casing smaller

This dimishes optical 'gaps' in the road at side streets
This was in particular visible at crossings of primary and secondary.
Render turning loops as turning circles

This closes #295.
Clean up turning circle code

Make order more logical.
This should not change the rendering.
Make fill of residential/unclassified/service tunnels grey

Make the fill of tunnels for highway types residential, unclassified,
and service tunnels light gray. The old dashed rendering was not
sufficient to make the distinction clear.
Fix residential/unclassified casing width on z13

This fixes two typos.
Clean up canal rendering.

This does not change rendering.
Do not render nature reserve names on z11

Currently names of small nature reserves clutter the map on z11.
This commit prevents that.
Fix extent

This solves #554