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.
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.
Make "base" landuse colours more consistent

"Base" landuse colours have some issues and are generally without a great deal of reason. As some examples

* Lightness is all over the place
* Of the "base" landuses only ``landuse=retail`` has a border, but it's strong-red, only it's got a width of 0.3 so you don't realize it.
* ``landuse=retail`` is more closely related to shop POIs than ``landuse=commercial``, but ``landuse=commercial`` has a colour more similar to the shop colours

A few things are done here

* Split out "base" solid landuse colours of residential, retail, commercial, industrial/railway, and farm/farmland.
* Gives them all the same lightness of 89. This is about the same as residential and retail were, and lightens the others, particularly commercial and industrial.
  * This basically flips retail and commercial in some ways. I looked at keeping them more the same, but I kept on tending to this, so decided to do it this way so that there was a colour difference between them
* Adjusts chromas
* Moves the industrial hue farther away from red. This is necessary with such light colours to allow it to be distinct from commercial while commercial is distinct from both retail and residential.
* Adds light lines to the ones missing them, consistent with current retail, school/university/college/hospital/kindergarten and most more POI-like landcover styles
  * Retail centres, industrial parks, residential development and commercial areas may all be named, and with a name is useful to have a border to indicate what size area the name applies to.
  * It aids in showing the limits between railway usage and adjacent industrial usage
  * Farm fields can be individually mapped, and the lines can be useful there, and those areas are typically low-clutter
  * The effect is intentionally fairly subtle
* Widened existing lines around retail, but still under 1 pixel. This helps with antialiasing artefacts. I didn't go all the way to 1, because I found that looked worse where there were two adjacent landuses of the same type.

It is possible to compute the difference between colours, either by transforming to Luv and using Euclidean distance, or [calculating directly in Lch](https://en.wikipedia.org/wiki/LCH_color_space#Color_and_hue_difference)

```
         Res.  Retail  Comm.  Ind.   Farml.
Farml.   12.0    12.4   10.0  17.3      0
Ind.      9.0    13.9    8.1   0
Comm.     8.5    7.6     0
Retail   16.0    0
Res.      0
```
Rework farmyard colour

Although not quite a "base" landuse in the same way farmland is, it's
worth handling at the same time to keep it in harmony with farmland.
Make base landuse lines slightly narrower for unnamed landuses