Render building labels from the same zoomlevel as landuse labels

Render building labels from 3000 pixels.

Old minimum pixel sizes for labels:
1643 on z14 (very rare), 3505 on z15 and z16.

This makes the code more consistent.
Update icons from upstream

* Windmill, place of worship, hostel: use new icons.
* Bus station, camping: use icon of more consistent type.
* Cinema, recycling, hairdresser: minor cosmetic changes.

Used upstream icon repository at http://www.sjjb.co.uk/mapicons/contactsheet.
improve code readability
Prevent gaps between adjoining areas of same type

Solution by @pnorman: Exclude the extremely small and adjust polygon-gamma
for the large.

This resolves #672.
Render tertiary yellow from z12 instead of z13

This resolves #824.
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
stop rendering larger icons for unknows shops
stop rendering [tourism = bed_and_breakfast] - finish #695
merge definitions of similar labels
Render shop=mall label like landuse=retail label

* Resolves #759
* Resolves #797
Render nature-reserve and theme park overlays below text and icon layers

This resolves #1046.

This resolves #1050
Revert bus station icon
Create layer of low-priority symbols

This layer contains symbols that have lower priority than text, i.e. symbols
that should not prevent the rendering of text labels.

The layer currently contains the barriers, level crossings, and mini
roundabouts.

This resolves #1046.
Don't render a cross on place of worshop with denomination=jehovahs_witness

* This resolves #1058

Thanks to @donaciano for the initial version in #75.
Make highway track areas more consistent with tracks

Resolves #445
Render name along borders of huge national parks and nature reserves

This contributes to solving #69 and #563
Merge pull request #1059 from mkoniecz/clean

refactoring amenity-points
Merge pull request #1075 from math1985/low-priority-symbols

Create layer of low-priority symbols
Merge branch 'jehovah-witnesses' of https://github.com/math1985/openstreetmap-carto into math1985-jehovah-witnesses

Conflicts:
	project.mml
Merge branch 'math1985-jehovah-witnesses'